## 4 Discussion
### Comparisons to Other Friction Stir-Based Repair Approaches  
Comparing the preliminary results of additive friction stir deposition in this work to other friction stir-based repair approaches--such as friction taper plug welding, filling friction stir welding, and refill friction stir spot welding (RFSSW) [13; 16; 23]--the latter are currently more mature, producing remarkable repair quality. Most notably, RFSSW has been demonstrated to successfully fill 7 xxx aluminum through-holes of similar sizes to that in the present study [23]. While the final microstructure in the repaired volume is similar in additive friction stir deposition and RFSSW, RFSSW demonstrates excellent mixing throughout the depth of the hole, without any gaps observed at the interface between the plug and the hole wall. Comparable repair quality and interfacial mixing are achieved in Experiment 1, but the single hole filling in Experiment 2 and the groove filling in Experiment 3 show inadequate mixing in the lower portions of the repair.  
Despite the imperfect repair quality demonstrated in Experiment 2 and Experiment 3 in the present work, the potential of large-volume repair using additive friction stir deposition should never be underestimated. First, additive friction stir deposition is able to robustly add material, so it is flexible with the repair geometry, whether it is a keyhole, long trench, or wide groove. The plug-based friction stir approaches, while enabling sufficient mixing and good repair quality, are limited by their repair geometry, not to mention the additional need of fabricating plugs of specific sizes and shapes. It would be quite challenging, if possible at all, to use these approaches to repair the long, wide groove shown in Experiment 3. Second, the results presented in this work represent the first attempts at using additive friction stir deposition to repair AA 7075. No optimization work has been performed  
Figure 9: (**a**) Illustration of the deformation zones in additive friction stir deposition; (**b**) illustration of the interaction between the deposited material and the hole edges during hole filling using additive friction stir deposition.  
regarding the deposition parameters (e.g., traveling speed and rotation rate), the dwell time, or the repair strategy (e.g., single vs. multiple tracks of deposition). Furthermore, the tool geometry can be tailored to the specific shapes and sizes of damages, and tool design will likely play an important role in large-volume repair in any future research of additive friction stir deposition. With systematic optimization work on these aspects, additive friction stir deposition should enable much improved repair quality than is demonstrated in the present work.  
We note that the fine grain microstructure in the repaired volume is a result of additive friction stir deposition that may affect other properties of the component, such as corrosion resistance. Both positive and negative effects of small grain sizes by friction stir processing have been reported in literature on the corrosion performance [36; 37; 38]. This aspect remains to be explored for additive friction stir deposition-based repair.