Interfacial Microstructure and Formation of Direct Laser Welded CFRP/Ti-6Al-4V Joint

Round 1

Reviewer 1 Report

This paper  presented an experimental investigation on joint properties of CFRP/Ti-6Al-4V Joint by direct laser welding. It was concluded that the joint strength increased when the welding speed is decreased up to 10mm/s. However, the joint strength decrease when the welding speed is further decreased. It was also concluded that good quality joint can be achieved by this method. EDS, X-ray and XPS test were conducted and formation of Ti-O-C bond was observed. The results are interesting and will attract readers. However, the following needs to be addressed before publication:

1. A massive English editing is necessary.
2. Why some authors name were highlighted? Also, why 'and' is used in the middle of the authors name. 'and' should be used just before the last author.
3. In the abstract: line 13-14: Please rewrite the sentence as below: "As the welding speed decreased, the tensile shear first increased and reached maximum value 926N at the speed of 10mm/s. The joint strength decreased when the welding speed is decreased further".
4. Page 1 line 43-45: Please include the self piercing riveting as other joining methods and include the following reference : Quality of self-piercing riveting (SPR) joints from cross-sectional perspective: A review- Archives of Civil and Mechanical Engineering Volume 18, Issue 1, Pages 83 - 93January 2018.
5. Line 191: Please give a space like : 'Figure 5'
6. Line 254: Please give a space like : 'Table 3. Element content of spot A in Figure 8'
7. Figure 2a: Please provide the arrow for the welding direction.
8. How many samples were tested for each of the welding speed to achieve the errors shown in figure 2f.
9. Why only the joint characteristics for the speed of 12, 10 and 8mm/s are shown in figure 3? What are the joint characteristics for the speed of 16 and 14mm/s?
10. As the highest joint strength was observed for 10mm/s speed it will be good to further extend the experiment for the welding speed of 6mm/s to show the trend is correct up to 10mm/s and further decrease in welding speed resulted in decreased joint strength.
11. Please include the joint characteristics for 16, 14 and 6mm/s in figure 5.
12. Figure 7 caption : please specify that the interface was for the joint of  10mm/s to make the figure standalone.
13. No mention of figure 8 in the text.
14. Table 3 caption: Is it for figure 7 or figure 8? Please make necessary corrections.
15. Table 3: What is the meaning of C K, O K, Al K and Ti K?

Author Response

Dear editor

I am grateful for your advice and comments in my paper. This paper had been revised with the “purple color” highlighted in the article and some questions would be answered as follows according to your comments. The point-to-point responses are presented as followed:

1.A massive English editing is necessary.

2.Why some authors name were highlighted? Also, why 'and' is used in the middle of the authors name. 'and' should be used just before the last author.

3.In the abstract: line 13-14: Please rewrite the sentence as below: "As the welding speed decreased, the tensile shear first increased and reached maximum value 926N at the speed of 10mm/s. The joint strength decreased when the welding speed is decreased further".

4.Page 1 line 43-45: Please include the self piercing riveting as other joining methods and include the following reference : Quality of self-piercing riveting (SPR) joints from cross-sectional perspective: A review- Archives of Civil and Mechanical Engineering Volume 18, Issue 1, Pages 83 - 93January 2018.

5.Line 191: Please give a space like : 'Figure 5'

6.Line 254: Please give a space like : 'Table 3. Element content of spot A in Figure 8'

7.Figure 2a: Please provide the arrow for the welding direction.

8.How many samples were tested for each of the welding speed to achieve the errors shown in figure 2f.

Response:

(1) English editing had been revised.

(2) The authors had been revised.

(3) The sentence had been rewritten.

(4) The reference had been cited in this paper.

(5)~(7) The space and arrow had been added in appointed place.

(8) Five samples were tested in this experiment to achieve the errors. that had been added in the paper.

9.Why only the joint characteristics for the speed of 12, 10 and 8mm/s are shown in figure 3? What are the joint characteristics for the speed of 16 and 14mm/s?

10.As the highest joint strength was observed for 10mm/s speed it will be good to further extend the experiment for the welding speed of 6mm/s to show the trend is correct up to 10mm/s and further decrease in welding speed resulted in decreased joint strength.

11.Please include the joint characteristics for 16, 14 and 6mm/s in figure 5.

Response:

(9)~(11) The joint characteristics and fracture surface for 16mm/s and 14mm/s had been added in this paper according to your requirement. As the joint for 6mm/s had been serious damaged during the welding process, the samples were easily broken down before mechanical test in this parameter, presenting even no mechanical property. Therefore, the joint for 6mm/s is free of application due to the horrible joint appearance and poor mechanical property. However, the fracture surface of CFRP for 6mm/s in this paper were added to illustrate the situation.

12.Figure 7 caption : please specify that the interface was for the joint of  10mm/s to make the figure standalone.

13. No mention of figure 8 in the text.

14.Table 3 caption: Is it for figure 7 or figure 8? Please make necessary corrections.

15.Table 3: What is the meaning of C K, O K, Al K and Ti K?

Response:

(12) The interface had been marked in the figure 9.

(13)~(15) The figure labels and the mistakes in Table 3 had been revised.

Here are my responses to all the questions. Please check the points and thanks again for your kindly comments on my paper.

Best Regards.

Author Response File: Author Response.docx

Reviewer 2 Report

Abstract: there is a golden rule that often applies to all abstracts in the scientific literature: 1. First sentence in the abstract should always be a short (and relevant) summary that refers to the field author is writing about (introduction to the problem/field). 2. Second sentence should always include current state of knowledge (what is known or what is the biggest challenge in the field). 3. Third sentence should address this challenge and author should explain how her/his research tackles it - in other words author needs to explain how this particular paper contributes to the problem stated in sentence 2. Having said that, I would highly recommend to modify the structure of the abstract so it gives better introduction to the field.

Paragraph 88 and 90: What is the unit of your sample (mm or inch)?

Paragraph 114: Can author share some insights where the shear blade was applied on the joint structure while shear testing? In this type of test it is sometimes tricky to test performance in shear as bending might be a big contributor to the performance. Small illustration with shear setup could be beneficial for the reader.

Paragraph 181 and others: I strongly recommend to use normalised shear strength (N/mm) instead of shear force (N). This would potentially be more helpful for future readers to compare their results with these presented in this paper.

General  comments:
Can author share the sample size?

Was welding performed at 100% of laser power and was there any way to measure or even estimate the temperature reached during welding?

Author mentioned few other bonding techniques - would it be possible to compare shear performance with other techniques?

Author Response

Dear editor

I am grateful for your advice in my paper. This paper had been revised with the “yellow color” highlighted in the article and some questions would be answered as follows according to your comments. The point-to-point question and responses are presented as followed:

1.Abstract: there is a golden rule that often applies to all abstracts in the scientific literature: 1. First sentence in the abstract should always be a short (and relevant) summary that refers to the field author is writing about (introduction to the problem/field). 2. Second sentence should always include current state of knowledge (what is known or what is the biggest challenge in the field). 3. Third sentence should address this challenge and author should explain how her/his research tackles it - in other words author needs to explain how this particular paper contributes to the problem stated in sentence 2. Having said that, I would highly recommend to modify the structure of the abstract so it gives better introduction to the field.

2.Paragraph 88 and 90: What is the unit of your sample (mm or inch)?

3.Paragraph 114: Can author share some insights where the shear blade was applied on the joint structure while shear testing? In this type of test it is sometimes tricky to test performance in shear as bending might be a big contributor to the performance. Small illustration with shear setup could be beneficial for the reader.

4.Paragraph 181 and others: I strongly recommend to use normalised shear strength (N/mm) instead of shear force (N). This would potentially be more helpful for future readers to compare their results with these presented in this paper.

Response:

1.The abstract had been totally rewritten according to your advice.

2. The unit is “mm” that had been revised in the paper
3. The illustration for tensile test had been added in this paper as “Figure 1b”
4. The joint strength had been measured by “N/mm”

General  comments

1.Can author share the sample size?

2.Was welding performed at 100% of laser power and was there any way to measure or even estimate the temperature reached during welding?

3.Author mentioned few other bonding techniques - would it be possible to compare shear performance with other techniques?

Response:
1.The sample size had been shared in the “2.1 materials”

2.Actually, the laser welding system has included the recording system that had been added in the “equipment” in this paper. Therefore, the laser power set in this experiment can be in accord with the practical power.

3.Honestly, the strength of joint between the two materials formed by laser welding can not compared to that formed by adhesive joint and mechanical property at this stage. However, we conduct this research because joint by laser welding process is a promising manufacturing process that can get a perpetual joint, and we are on the way to develop the process to improve the strength of joint, although the joint formed by direct laser cannot reach to ideal status at early stage.

Here are my responses to all the questions. Please check the points, thanks again for your kindly comments on my paper.

Best Regards.

Author Response File: Author Response.pdf

Reviewer 3 Report

Good article. Few comments though.

1. English check is required.
2. The experimental section needs a bit of addition of information like focal length, collimator size, Rayleigh length, gas flow rate etc.
3. It would be good if you could provide a plot showing effect of heat input on depth and width of the weld.
4. Fig. 3 caption should explain which image is what.

Author Response

Dear editor

Thank you for your advice and comments for my paper and I am writing this letter to make a response to your comments. This paper had been revised with the “blue color” highlighted in the article and some questions would be answered as follows according to your comments. The point-to-point responses are presented as followed:

Comments:

1.English check is required.

2.The experimental section needs a bit of addition of information like focal length, collimator size, Rayleigh length, gas flow rate etc.

3.It would be good if you could provide a plot showing effect of heat input on depth and width of the weld.

4.Fig. 3 caption should explain which image is what.

Response:

(1) English editing had been revised.

(2) The additional information had been added in this article in the chapter of experiment highlighted by blue color.

(3) The plot of effect of the welding parameter on depth and width of weld at CFRP side was added in Figure 3b. As the Titanium alloy was thin and was almost penetrated during the welding process, the width of weld was collected.

(4) The caption for the microstructure of joint had been explained in this paper.

Here are my responses to all the questions. Please check the points and thanks again for your kindly comments on my paper.

Best Regards.

Author Response File: Author Response.pdf

Reviewer 4 Report

The paper entitled “Interfacial Microstructure and Formation of Direct Laser Welded CFRP/Ti-6Al-4V Joint” by Pengyuan et al deals with the laser welding of CFRP-PEEK/Ti6Al4V. Influence of laser welding was evaluated. Microstructure and mechanical properties of the welds were evaluated.

Results are in the scope of the Metals journal. These are interesting, but after reading the paper, I have some comments about it:

 

GENERAL COMMENTS:             

• In general, this paper is well-written and very interesting; however, I see two main problems: (a) authors should clarify if the shear strengths reached in this work are useful (and the potential applicability of them; e.g. are these strengths suitable to be used in the automotive industry?); (b) authors perform an interesting effort to characterize the element distribution with SEM-EDS, XRD and XPS; but, which are the main results? A lot of data are provided but the utility of this is not clear. Is possible to state that Ti-O-C bonds are present? How does the presence of the phases: CTi0.42V1.58 , TiO2, TiO, TiC affect to the weld performance? I think authors should discuss these results more carefully in the manuscript.
• Please, revise all the captions for most of the figures. It is not clear the processing conditions associated to each image within a figure (e.g. Figs. 3,5,7,8,9). On the other hand, some images (e.g. those to XRD, SEM-EDS and XPS) are not associated to any sample (i.e. to any processing condition). Please, improve this.
• English language should be revised.

 

PARTICULAR COMMENTS:

• (Page 1, Line 14) Please, replace “… and reached maximum value 926N…” with “… and reached a maximum value of 926N…”
• (Page 1, Lines 36-37) Please, replace “Titanium alloy with superb mechanical properties and corrosion resistance is widely used in the aviation field” with “Titanium alloys with superb mechanical properties and corrosion resistance are widely used in the aircraft industry”
• (Page 3, Line 94) Please, replace “The CFRP plate was Rinsed and dried…” with “CFRP plates were rinsed and dried…”
• (Page 3) Please add the focal length, and the flow rate used for the protective gas. Why do you use only one laser power? Clarify it in the text.
• (Page 3) Please, provide more details about the mechanical tests. Which is the shape of the test coupons? Was any standard used for the testing?
• (Page 5) Please, improve the caption of figure 3. It is not clear which are the processing conditions for each image.
• (Page 5, Line 162) Please, replace “As shown in Figure d-f…” with “As shown in Figure 3d-f…”
• (Page 5, Line 170) Please, replace “were smooth and plat while” with “were smooth and flat while”
• (Page 6, Line 185) Please, replace “to 703N which meaned that” with “to 703N which meant that”
• (Page 6) The explanation of the influence of the welding speed in the shear strength is not very clear and convincing. Authors state “when the welding speed decreased, the melted area increased but cracks and other defects were formed in the weld area.”; however, the Figure 4 shows the opposite trend. As the welding speed is reduced, the shear strength is increased. It seems not very real that increasing defects (when the welding speed is increased), improves the shear strength. Please, revise carefully this part.
• (Pages 8-9) Notice that the data of the elemental distribution given by EDS is only an estimation (unless, EDS is done in a quantitative way through comparison with standards). On the other hand, authors cannot rely only on EDS results to estimate the amount of O or C. O and C can have an external origin, such as contamination of the samples… In summary, EDS is a good method to obtain a semiquantitative estimation of the elements (unless performed with a comparison with standards), and it is not a. good method to estimate the presence of O, C and similar elements.

Author Response

Dear editor

I am grateful for your advice in my paper. This paper had been revised with the “green color” highlighted in the article and some questions would be answered as follows according to your comments. The point-to-point responses are presented as followed:

GENERAL COMMENTS:

1.In general, this paper is well-written and very interesting; however, I see two main problems: (a) authors should clarify if the shear strengths reached in this work are useful (and the potential applicability of them; e.g. are these strengths suitable to be used in the automotive industry?); (b) authors perform an interesting effort to characterize the element distribution with SEM-EDS, XRD and XPS; but, which are the main results? A lot of data are provided but the utility of this is not clear. Is possible to state that Ti-O-C bonds are present? How does the presence of the phases: CTi0.42V1.58 , TiO2, TiO, TiC affect to the weld performance? I think authors should discuss these results more carefully in the manuscript.

Response:

(1) Honestly, the strength of joint between the two materials formed by laser welding cannot compared to that formed by adhesive joint and mechanical property at this stage. However, we conduct this research because joint by laser welding process is a promising manufacturing process that can get a perpetual joint, and we are on the way to develop the process to improve the strength of joint in the future, although the joint formed by direct laser cannot reach to ideal status at early stage.

2.Please, revise all the captions for most of the figures. It is not clear the processing conditions associated to each image within a figure (e.g. Figs. 3,5,7,8,9). On the other hand, some images (e.g. those to XRD, SEM-EDS and XPS) are not associated to any sample (i.e. to any processing condition). Please, improve this.

Response:

(2) Actually, the test to reveal the interface structure is quite difficult in this study because the reaction layer was too thin to identify and the normal test for weld like the EDS and XRD cannot directly analyze the structure independently. We conducted EDS, XRD and XPS test in this study to reveal the interfacial structure as roundly as possible. Firstly, we utilized the EDS only to identify the elements at the interface. Then, XRD test on the surface of weld at titanium alloy side could only reveal the phases that had a certain amount in this study, while the other composites with minor amount could not be indicated in this test. Finally, XPS test was a further way to figure out the chemical bond that could identify the nanoscale composite. Consequently, the results of the tests were not in conflict but in complement with each other. However, it is still a challenge to further reveal the structure deeply and TEM test may be used in our study in the future.

In addition, the tests were only tested on the sample at the welding speed of 10mm/s just to demonstrate the chemical bonding that can form the joint. Our group decides to further test other samples at different parameter to reveal the relationship between mechanical property and interfacial structure in the future.

3.English language should be revised.

Response:

(3) The captions had been revised and English language had been revised roundly.

PARTICULAR COMMENTS:

• (Page 1, Line 14) Please, replace “… and reached maximum value 926N…” with “… and reached a maximum value of 926N…”(revised)
• (Page 1, Lines 36-37) Please, replace “Titanium alloy with superb mechanical properties and corrosion resistance is widely used in the aviation field” with “Titanium alloys with superb mechanical properties and corrosion resistance are widely used in the aircraft industry” (revised)
• (Page 3, Line 94) Please, replace “The CFRP plate was Rinsed and dried…” with “CFRP plates were rinsed and dried…”(revised)
• (Page 3) Please add the focal length, and the flow rate used for the protective gas. Why do you use only one laser power? Clarify it in the text. (revised by green color)
• (Page 3) Please, provide more details about the mechanical tests. Which is the shape of the test coupons? Was any standard used for the testing? (revised by green color)
• (Page 5) Please, improve the caption of figure 3. It is not clear which are the processing conditions for each image. (revised)
• (Page 5, Line 162) Please, replace “As shown in Figure d-f…” with “As shown in Figure 3d-f…”(revised)
• (Page 5, Line 170) Please, replace “were smooth and plat while” with “were smooth and flat while” (revised)
• (Page 6, Line 185) Please, replace “to 703N which meaned that” with “to 703N which meant that” (revised)
• (Page 6) The explanation of the influence of the welding speed in the shear strength is not very clear and convincing. Authors state “when the welding speed decreased, the melted area increased but cracks and other defects were formed in the weld area.”; however, the Figure 4 shows the opposite trend. As the welding speed is reduced, the shear strength is increased. It seems not very real that increasing defects (when the welding speed is increased), improves the shear strength. Please, revise carefully this part.( (revised by green color)
• As mentioned above, the EDS test in this study was to identify the elements that existing at the interface. The O and C element were actually concentrated at the interface. Therefore, the XPS test were used to ensure the composites that might formed by O and C elements.

Here are my responses to all the questions. Please check the points, thanks again for your advice and comments on my paper.

Best Regards.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Authors of this manuscript made significant effort to address the reviewers comments. It is good to go now.

Author Response

Dear editor

Thanks again for your advice and comments for my paper, and I am grateful for your support on our study. We will complete our research and step forwards to make our study perfect.

Best Regards.

Author Response File: Author Response.pdf

Reviewer 4 Report

In the current state, the paper entitled “Interfacial Microstructure and Formation of Direct Laser Welded CFRP/Ti-6Al-4V Joint” by Zou et al is suitable for publication. Authors have addressed all the comments made by the reviewer; however, prior to publication, I recommend to make the following changes in the text:

• (Line 13) Please, replace “difference of material property is the biggies challenge to join them” with “difference of material properties is the biggest challenge to join them”
• (Line 13) Please, replace “Continues laser welding” with “Continuous laser welding”
• (Line 15) Please, replace “Mechanical property and” with “Mechanical properties and”
• (Line 97) Please, replace “The CFRP plates were Rinsed” with “CFRP plates were rinsed”
• (Line 111) Please, replace “Laser process parameters were shown” with “Laser process parameters are shown”
• (Line 158) Please, replace “weld morphology of titanium” with “Weld morphology of titanium”
• (Line 189) Please, replace “the weld profile and” with “The weld profile and”
• (Line 201) Please, replace “the relationship between” with “The relationship between”
• (Line 231) Please, replace “the fracture surface” with “The fracture surface”

Author Response

Dear editor

Thank you for your advice and comments for my paper and I am writing this letter to make a response to your comments. This paper had been revised with the “grey color” highlighted in the article and some questions would be answered as follows according to your comments. The point-to-point responses are presented as followed:

Comments:

• (Line 13) Please, replace “difference of material property is the biggies challenge to join them” with “difference of material properties is the biggest challenge to join them”
• (Line 13) Please, replace “Continues laser welding” with “Continuous laser welding”
• (Line 15) Please, replace “Mechanical property and” with “Mechanical properties and”
• (Line 97) Please, replace “The CFRP plates were Rinsed” with “CFRP plates were rinsed”
• (Line 111) Please, replace “Laser process parameters were shown” with “Laser process parameters are shown”
• (Line 158) Please, replace “weld morphology of titanium” with “Weld morphology of titanium”
• (Line 189) Please, replace “the weld profile and” with “The weld profile and”
• (Line 201) Please, replace “the relationship between” with “The relationship between”
• (Line 231) Please, replace “the fracture surface” with “The fracture surface”

Response:

All the capital and small letters had been revised in this paper, and obvious grammar problems had been modified highlighted by grey color.

Here are my responses to all the questions. Please check the points and thanks again for your kindly comments on my paper.

Best Regards.

Author Response File: Author Response.pdf