In Situ and Post-Mortem Characterizations of Ultrasonic Spot Welded AZ31B and Coated Dual Phase 590 Steel Joints
Round 1
Reviewer 1 Report
The study represents an interesting investigation of the ultrasonic spot welding to produce dissimilar joints. The manuscript can be improved by considering the following:
- A detailed literature review is needed to bring out the need for the present work. At present, authors have mentioned few studies related to laser welding, RSW, FSW and USW. Authors need to justify the advantages and challenges of each joining technique mentioned here. How the USW can be used to overcome these challenges.
- A comprehensive review is needed to identify the gaps in the literature and novelty of this work.
- Please clarify with an example where this joint can be used within the automotive structure. The current state-of-the-art joining techniques used to produce these joints within automotive industries.
- Please check the figure and table references within the text.
- Please clarify the description of inferred camera use in this study. It is well-known that the emissivity of metal is low and whether authors uses any special treatment to measure the temperature rise.
- Authors need to clarify the amplitude of ultrasonic vibration used in this study. When both Sonotrodes were with 180Ëš phase shift from each other the welding energy may be much higher than single Sonotrode vibration. Please explain the basis for the selection of these process parameters (i) set-up 1 for 1000W for 0.5s, and (ii) set-up 2 much higher energy 3500W for 4s.
- The indentation can better be explained with micro-hardness measurement and their behaviours during the welding.
- Please provide the details of DIC measurement, at least the image resolution and high-speed frame rate were used in this study.
- Authors need to clarify the reasons for interface failure at higher load and parent material failure at lower load during fatigue testing.
- Please quantify the Zn and Zn-Mg reaction layers as shown in Figure 5 and 6. Also, compare with the as-received Zn thickness.
- Please concise the conclusions point-wise identifying the key points.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
An interesting study for the disclosure of ultrasonic welding bonding process. Only a couple of things need to be confirmed.
1. In Fig. 1, the authors provided the information regarding the relative velocity between the upper sonotrode/DP590/AZ31B/bottom sonotrode interfaces. With respect to the relative displacement, was there any significance at those interfaces? It is reasonable that the relative displacement between steel/Mg could be significantly changed from the beginning to the achievement of the bonding process. A brief explanation, if available, is helpful for the readers' better understanding.
2. If possible, please provide the oscillation amplitude of the apparatus of this experiment as a reference for the relative motion between workpieces.
3. In the microstructural analysis section, the authors provided the interfacial microstructure (SEM image) and corresponding elemental map results, indicating the Zn-Mg eutectic reaction layer. Is there any difference in the atomic percentage of eutectic phase of reaction layer (e.g., Zn-Mg layer in Fig. 5 and the matrix phase in Zn-Mg layer in Fig. 6) from the EDS analysis?
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Authors have addressed the comments. It is acceptable now.
Reviewer 2 Report
The authors made appropriate modifications towards the comments, such as differences in the relative displacement, and microstructure characterization of the reaction layers. With those additional discussions, the manuscript would be recommended for the publication.
