Effect of PWHT on the Microstructure and Mechanical Properties of Friction Stir Welded DP780 Steel
Round 1
Reviewer 1 Report
The study is mainly focused on the effects produced by thermal treatments on the microstructure of FSW joints. Concerning friction stir welding process, the experimental tests are restricted to two conditions (transverse speed equal to 200 mm/min and 400 mm/min, assuming fixed values for the other machining parameters). For this reason, the friction stir welding process appears marginal while the analysis is focused on benefits of PWHT of DP steels. However, even in this case, the experimental plan appears to be lacking as the tests were carried out taking into account few experimental conditions. Finally, no information is provided on the number of replicates. The work done fulfils the aims and scope of Metals and it is suggested that the manuscript could be considered for publishing after the following comments are taken care of.
- Selection of welding parameters adopted in experimental tests (i.e. rotation speed equal to 400 rpm and travelling speed equal to 200 and 400 mm/min) determines N/V (ratio of rotation speed to travelling speed) values between 1.0-2.0, while optimal values are between 2.2-4.2. Moreover, Kim et al. conducted FSW tests for cold rolled DP590 steel sheet adopting a tool rotation speed of 600-1000 rpm and travelling speed of 120-420 mm/min. Concerning FSW parameters, can the authors better explain their choice?
- Raw 60: “of the” is repeated twice.
- Raw 116: Add the pin length value.
- Raw 124: What condition does top-view shown in Figure 1c refer to? The FSW400-200 one? Is it possible to add top-view for FSW400-400 sample? Are there any differences? Explain the observations in rows 150-154.
- Table 1 - Add percentage (%) to values.
- Raw 128: verify the sentence: “Thermos-mechanically affected zone”.
- Raws 172-173: Are there differences between FSW400-200 and FSW400-400 in the SZ?
- Raws 174-175: Caption of Figure 2 is not clear. It is recommended to put letters (a, b, … f) on all images. It’s useful to add magnification values too.
- Raw 177: “The average hardness value of DP780 steel was about 295 HV and 273 HV for FSW400-400 and FSW400-200, respectively”, but what are the standard deviation values? And what is the BM hardness value?
- In Figure 3b, explain the two graphs where hardness is reported as a function of elastic modulus.
- Does data reported in Table 3 refer to curves depicted in Figure 4? For example, εT = 23,4% in Table 3, while εT < 20% in Figure 4?
How many tests have been conducted for FSWed joints? Are data shown in Table 3 average values? Is it possible to add standard deviation values?
- Raw 201: the sentence:“These values are similar to those of the base material”, should be verified adding standard deviation values.
- Raws 221-223: By observing Figure 6, it isn’t clear the affirmation: “boundaries between the weld and BM zones resemble the geometry of the tool tip and the shoulder”.
- Is magnification of Figure 6a equal to the one of Figure 6b? Adding a marker in Figure 6a can be useful.
- It would be also useful to report the microstructure showing the interfaces between SZ, TMAZ, and HAZ for FSW400-200 sample in order to analyze differences and/or similarities with the FSW400-400 one.
- Figure 7 – Table 4: it isn’t clear why FSW400-200 joints produce higher strain (εU (%) and εT(%)) values with respect to FSW400-400 ones, when annealing process was carried out at 650 °C.
- Raw 318: verify the sentence: “ductility degreased”.
Author Response
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Author Response File: 
Author Response.pdf
Reviewer 2 Report
Dear Authors,
Please find the pdf with my comments in it.
Thank you.
Comments for author File: Comments.pdf
Author Response
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Author Response File: Author Response.pdf
Reviewer 3 Report
Dear authors,
This paper is a very interesting paper on the "Effect of PWHT on the microstructure and mechanical performance of FSW DP780 steel", but we would like you to reconsider the following points.
(m) mandetry
(o) optional
[1] (m) Abstract, p. 1, LL. 16-17 "Lower strain elongation and higher yield and ultimate strength strengths were recorded for as-welded samples as compared to DP780 steel.": I think that the meaning of "compared to DP780 steel" means comparison with the base material. Since all the test materials in this paper are DP780 steel, we think this expression should be "as compared to the base material".
[2] (m) Chapter 1, p. 1, L. 34 "inter-critical annealing": This expression is not very familiar. Please reconsider this expression.
[3] (m) Chapter 2: Which is the DP780 steel used as the test material, rolled or extruded? Please specify in the text.
[4] (m) Chapter 2, p. 3, Table 1: Is this chemical composition mass% or mol%? Please enter the unit in the table or text.
[5] (m) Chapter 2, p. 3, Fig. 1 (a): Please, insert the length of the parallel part of the tensile test piece and the radius of the R part (shoulder part).
[6] (m) Chapter 2, p. 3, "welding tool": Please, describe the pin length of the joining tool in the text.
[7] (m) Section 3.1, Fig. 1 (c): Isn't the definition of the direction of the top "weld seam surface" in Fig. 1 (c) the WD-TD direction?
[8] (m) Section 3.1: Comparison is impossible unless the BM hardness is stated in the manuscript.
[9] (m) Table 3: σy should generally be shown by proof stress. In addition, the values ​​in Table 3 should be expressed as statistically processed values.
[10] (m) Fig. 6 (b) caption: Fig. 6 (b) does not show the mechanism of AGG, so it is better to change the caption.
[11] (m) Section 3.2, p. 8, LL. 240-241 "As shown, the FSWed DP780 breaks in SZ,": In Section 3.1, the FSWed sample was damaged away from the weld area between HAZ and BM. It is described that it will occur (p. 6, LL. 201-204). Please reconfirm that the description of the rupture position is inconsistent.
[12] (m) Figure 9: The unit of hardness in Fig. 3 and the hardness unit in Fig. 9 are different, so redraw with the same unit, please.
[13] (m) References: Please check "Instructions for Authors" carefully for the format of references.
https://www.mdpi.com/journal/metals/instructions
[14] (m) Others: Others were written directly in the manuscript. Please refer to it when making corrections.
Best regard,
Comments for author File: Comments.pdf
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Dear Editor,
I believe the manuscript has been
sufficiently improved to warrant publication in Metals.
King regards,
A. Paoletti
Author Response
Authors would like to express their gratitude for your fruitful comments.
Reviewer 3 Report
Dear authors,
This paper is a very interesting paper on the "Effect of PWHT on the microstructure and mechanical performance of FSW DP780 steel", but we would like you to reconsider the following points.
(m) mandetry
(o) optional
[1] (m) Figure 4: It is considered that clear yield stress cannot be determined from Fig. 4. I think that proof stress should be used instead of σy in such a stress-strain curve.
[2] (m) Section 3.1: Please consider in the text why the rupture position of FSWed DP780 ruptured at SZ instead of the softening HAZ part where the strength is weak.
[3] (m) References: Please check "Instructions for Authors" carefully for the format of references. Journal names are to be abbreviated.
1. Author 1, A.B .; Author 2, C.D. Title of the article. Abbreviated Journal Name Year, Volume, page range.
https://www.mdpi.com/journal/metals/instructions
Best regard,
Author Response
Please see the attachment.
Round 3
Reviewer 3 Report
This paper is a very interesting paper on the "Effect of PWHT on the microstructure and mechanical performance of FSW DP780 steel", but we would like you to reconsider the following points.
(m) mandetry
(o) optional
[1] (m) 0.2% proof stress symbol: σ0.2 is generally considered as the notation of 0.2% proof stress.
[2] (m) References:
The format of the references by "Instructions for Authors" is specified as follows. https://www.mdpi.com/journal/metals/instructions
Journal Articles:
1. Author 1, A.B .; Author 2, C.D. Title of the article. Abbreviated Journal Name Year, Volume, page range.
The Journal Name in your manuscript is not Abbreviated Journal Name.
Best regard,
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
