**6. Outlook**

Pore formation in welds in nodal and centred position probably results from keyhole-disturbance, which will be investigated by optical coherence tomography subsequently. For exact conclusions regarding porosity distribution scanning acoustic microscopy will be used in future and for statistical coverage more experiments have to be conducted.

The temperature distribution over the specimen and the whole ultrasonic system will be investigated because ultrasonic excitation brings in much heat depending on the position in the amplitude distribution. This a ffects solidification.

As mentioned in Section 5, welding positions close to the antinode position should be investigated. Welds at centred position shows stronger melt shifting e ffects than antinode position and no V-collapse effect so that it could be used to segregate the e ffects of V-collapse including gas escapement through the keyhole in antinode position from the melt shifting e ffect, which also e ffects antinode position. Finally, the exact mechanism of flow directing will be investigated by simulations regarding the ultrasonic wave behaviour during passing the weld.

**Author Contributions:** Conceptualization, J.G., C.N., S.N., J.H., J.T., J.W. and S.K.; methodology, J.G., C.N. and S.N.; software, C.N.; validation, J.G., C.N. and S.N.; formal analysis, J.G., C.N. and S.N.; investigation, J.G., C.N. and S.N.; resources, J.G., C.N. and S.N.; data curation, J.G., C.N. and S.N.; writing—original draft preparation, J.G., C.N. and S.N.; writing—review and editing, J.G., C.N., S.N., J.H., J.T., J.W. and S.K.; visualization, J.G. and C.N.; supervision, J.W. and S.K.; project administration, S.N., J.T. and J.H.; funding acquisition, J.W. and S.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** Funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)-CRC 1153, subproject A3–252662854.

**Acknowledgments:** The authors would like to thank the German Research Foundation (DFG) for the financial and organisational support of this project.

**Conflicts of Interest:** The authors declare no conflict of interest.
