## 3 Results and Discussion
### Mechanical Properties  
When compared to the AA/7075-T6 rod BM, the variation in hardness along the continuously deposited multilayered portions may provide a better prediction of the mechanisms involved in the deposited layers that result in loss or increase in strength. The hardness values are controlled by the grain size structure and the presence of intermetallics [65; 66]. Figure 14 illustrates the hardness variation maps of the AA7075 BM and DP processed at a constant rotation speed of 400 rpm and feeding rats at 1, 2, 3, and 4 mm/min. The AFSD procedure resulted in a more homogeneous hardness distribution across the layers of the deposited materials (Figure 14b-e) than the AA/7075-T6 rod material (Figure 14a). In addition, within the DPs, the average hardness increased with increase in the deposition rate from 1 to 3 mm/min. This was followed by a slight decrease at a deposition feed rate of 4 mm/min. These values were 126 \(\pm\) 5, 142 \(\pm\) 6, 165 \(\pm\) 5, and 149 \(\pm\) 5, with the DPs additively friction stir deposited at a constant rod rotation speed of 400 mm/min and 1, 2, 3, and 4 mm/min, respectively. These values were lower than those given with AA/7075-T6 BM. Similar results have been reported in different works when the T6 material was subjected to thermomechanical processes [62; 64; 67; 68; 69]. The DP processed at 3 mm/min attained 90% of the AA/7075 BM. The hardness of this alloy is controlled by two mechanisms. The first mechanism involves an increase in hardness-related grain size reductions. The morphology, size, and dispersion of the stable intermetallics reflect the second mechanism. These two mechanisms result in increased hardness. However, the hardness of all the produced DPs is lower than the AA/7075-T6 BM. This may be due to the fact that the AA/7075-T6 is a full precipitation hardened alloy; thus, when exposed to high temperatures during deposition, these precipitates either dissolve or coarsen, thus the hardness is reduced. But the contribution of saturated solid solution (SSS) intermetallic fragmentation and dispersion and grain refining cannot compensate for the loss of hardness due to precipitate dissolution and coarsening [41; 70; 71].  
Figure 14: Hardness map distribution of the (**a**) AA/7075-T6 BM amd the deposited AA/7075-T6 materials produced at different deposition rates of (**b**) 1, (**c**) 2, (**d**) 3, and (**e**) 4 mm/min and a constant CRRS of 400 rpm.  
Figure 15 illustrates the engineering stress-strain curve and the ultimate compressive strength for the AA7075 DPs parts at the studied deposition conditions. It appears that the compressive strength of the DPs increases with increase in the feed rate from 1 to 3 mm/min and then decreases at a 4 mm/min deposition feed rate. In comparison to alloy AA7075-T6 BM, they had noticeably greater compressive strengths and demonstrated the ability to withstand up to 30% strain without fracture. The aluminum alloy's compressive properties depend on the tested materials' microstructure. After the AFSD process, the ultimate compressive strength of the DPs was higher than the AA7075-T6 BM at the deposition rate of 3 mm/min, where the maximum compressive strength was 1320 MPa and for the as-received material was 910 MPa. This improvement in compressive strength of the deposited specimen is due to the grain refining and highly dispersed fragmented intermetallics of the DPs [68]. Finally, for the AA7075-T6 DPs, it can be said that the compressive strength is highly affected by grain refining, while the hardness is highly affected by the precipitates.