Sonotrodes for Ultrasonic Welding of Titanium/CFRP-Joints—Materials Selection and Design

Round 1

Reviewer 1 Report

This study investigated the effects of sonotrode geometry and materials selection on fabricating hard Ti and CFRP hybrid joints, and these results systematically gave some hints for selecting proper materials on joining hard materials with sufficient durability of sonotrodes. Overall the finding in this study made a great contribution to the state of the art, but some points can be further clarified.

1. The authors showed the strengths of joints obtained from using different sonotrodes with the same knurled pattern, and WFN and MC90 possessed a similar joint strength. Do this feature have any relationship to the microstructure configuration of sonotrode material shown in Fig. 9 and Table 4? (e.g., grain and precipitation sizes) 
2. The strength comparison of Ti/CFRP joints fabricated by different sonotrodes (in material and tip geometry) is provided by the authors. A short comparison for these strength values of Ti/CFRP to other previous works (e.g., USW of Al/CFRP) and their failure modes (e.g., interfacial failure or pull-out nugget failure) can be added to provide readers a basic understanding for strength improvement by using these different kinds of sonotrode.

Author Response

Dear Reviewer

Thank you very much for your constructive comments. Please find our point-by-point response to your comments below:

1. The correlation between the microstructure of the sonotrode and the joint strength has not been investigated. Therefore, we did not discuss this point. As the microstructure of the sonotrode affects its hardness and strength, there might be an influence, but we do not have sturdy indications for that.
2. Further information and a comparison with other hybrid joints has been added in lines 476 and 483-487.

Best regards

Moritz Liesegang

Reviewer 2 Report

This is an interesting paper on developing sonotrodes for ultrasonic welding of titanium. The following concerns should be addressed before further consideration.

1. It is suggested to give out the chemical composition of the 3 different tool materials, RexM4, WFN and MC90 in a table.
2. It is suggested to add more references on ultrasonic welding of titanium, for example, DOI:10.1007/s11661-019-05395-7There are some papers on dissimilar ultrasonic welding of titanium with aluminum alloys including soft 6111 and high strength 2139 Al alloy. 
3. It is suggested that the authors could discuss the interfacial reaction between the tool and titanium, such as the formation and growth of intermetallic compounds at the interface and their relationship with tool life, the effect of welding temperatures on the interfacial reaction. The formation of brittle intermetallic compounds on the interface will probably reduce tool life.

Author Response

Dear Reviewer

Thank you very much for your constructive comments. Please find our point-by-point response to your comments below:

1. The chemical compositions have been added in tables 2-4
2. Further information has been added in lines 49-51.
3. The formation of IMC´s at the surfaces of the sonotrodes and the titanium has not been investigated in this work. A reduction of tool life due to abraded sonotrode particles or IMC´s that might occur is likely, but as we do not have any indications or data, we did not discuss that point in this work.

Best regards

Moritz Liesegang