## 3 Results and Discussion
### Macrostructure and Microstructure Investigation  
Macrostructure examination of the mid-section revealed sound, defect-free, continuously deposited multilayer build-ups without any obvious separation between the layers along the building direction and no noticeable cracks or voids through the interface between these layers and the substrate in all the produced DPs at the deposition conditions of 400 rpm and feed rates from 1 to 4 mm/min (Figure 8), indicating suitable proposed heat input to achieve defect-free deposited multilayers jointed well with the substrate plate. Figure 7: Thermal cycle of the AFSD process against the deposition time for the DPs processed at 400 rpm and different feed rates of (**a**) 1, (**b**) 2, (**c**) 3 and (**d**) 4 mm/min. Optical micrographs of the as-received AA7075 consumable rod, the AA2024 substrate plate and the AA7075/AA2024 interface are shown in Figure 9. Both the grain structures were large and elongated in the deformed direction for the rod material (Figure 9a,b) and the substrate sheet (Figure 9c). The interfacial region between the AA7075 deposited material and the AA2024 substrate is shown in Figure 9d,e. The deposited AA7075/AA2024 substrate interface shows a curved profile, which can be related to the mutual plastic deformation of both the substrate surface and the deposited material, which might involve mixing of the two material alloys and structure linkage. It was reported [1] that this curved interface indicates severe plastic deformation between the substrate surface and the deposited layers. Furthermore, the corresponding structural interlocking is predicted to result in intense material flow, good mixing, and significantly stronger interfacial adhesion. In the AFSD process, the interfacial adhesion between the deposited material and the substrate is crucial for the overall integrity and performance of the joint [48]. Structural interlocking refers to the mechanical interlocking of the material's micrometer scale across the interface, which enhances the adhesion strength [49]. During the AFSD process, the rotating tool stirs the material at the interface, resulting in intense plastic deformation and mixing of the material from the substrate and the deposited material. This mixing promotes the adhesion of the deposited layers to the substrate at the atomic scale. The structural interlocking helps in minimizing interfacial defects, such as voids, porosity, and inclusions. The intense plastic deformation and mixing during AFSD promote the redistribution and elimination of these defects, leading to improved interfacial quality and adhesion. Furthermore, the formation of strong metallurgical bonds at the interface due to the mixing of atoms and the refinement of the microstructure facilitate atomic diffusion and solid-state bonding, resulting in a larger contact area and better continuity between  
Figure 8: Macrographs of the AA7075 DPs processed at deposition feed rates of (**a**) 1, (**b**) 2, (**c**) 3 and (**d**) 4 mm/min and at a fixed CRRS of 400 rpm. the deposited material and the substrate with enhanced interfacial adhesion and improved load-carrying capacity [50]. The AFSD is a thermo-mechanical process, like both FSW [18; 51; 52; 53; 54] and FSP [17; 55; 56; 57], in terms of plastic deformation in the deposition zone associated with the heat generation. The temperature in the deposition zone may reach a value ranging from 60 to 80% of the melting point of the consumable rod BM. Thus, the continuously deposited multilayers suffer from a relatively high temperature and severe plastic deformation due to the stirring action under the continuous feed rate and the applied machine axial force. Thereby, the two possible parameters of continuous dynamic recrystallization [58] are established. The obtained microstructure shows significant grain refining compared to the AA7075 rod BM, as given in Figure 10. The DP grain size is noticeably influenced by the AFSD feed rate at a constant rod rotation speed.