*2.4. Metallographic Analysis*

Following the friction welding process, the samples were cut along the axis of rotation and the cross section of the joining zones were prepared for metallographic examinations by grinding, polishing and etching. The quality of the joining zone was analyzed on micrographs by detecting phases and inclusions. In addition, the Vickers hardness was measured according to DIN EN ISO 6507-1 [17] (HV0.1) to compare the mechanical properties of the joining zone and of the heat-affected zones with those of the base materials. Furthermore, the joining zone was analyzed by scanning electron microscopy (SEM) (AURIGA from Zeiss, Oberkochen, Germany) and energy dispersive X-ray spectroscopy (EDS) (Oxford Instruments, Abingdon, UK). A slope cut was prepared by applying a focused ion beam (FIB) to excavate a cross section not influenced by prior conventional steps of metallographic preparations.
