## 5 Conclusions  
To summarize, we have explored the use of additive friction stir deposition, an emerging solid-state additive manufacturing technology, for the repair of volume damages in AA 7075. The most salient conclusions from this work include:  
* Additive friction stir deposition proves to be effective at filling the entire volume of through-holes and wide grooves in AA 7075. This is especially noteworthy in the latter case, in which the width of the groove is 33% larger than the width of the feed rod.
* Additive friction stir deposition always enables sufficient mixing between the deposited material and the side wall of the hole or groove in the upper portions of the repair. This is indicated by a gradual transition from the elongated grains of the AA 7075 plate to the fine, equiaxed grains of the deposited AA 7075, showing no distinct interface.
* The repair quality of the lower portions is generally worse than the upper portions, sometimes showing straight, sharp interfaces separating the elongated grains and the fine, equiaxed grains. In through-hole filling, the depth for sufficient mixing is controlled by the interactions between the deposited material and the hole edges, which may involve the bending or even fracture of the hole edge pieces.
* The thermomechanical history in additive friction stir deposition generally leads to a slight decrease (<15%) in the hardness of the repaired volume as compared to the original feed material, although the peak hardness in the repaired volume can be comparable to the feed material.  
Overall, this work presents an initial investigation on the solid-state additive repair of AA 7075 using additive friction stir deposition, which shows promising results despite imperfect repair quality. With systematic optimization work on process control and repair strategies developing in the future, high-quality and highly flexible repairs are expected to be achieved using additive friction stir deposition.  
**Author Contributions:** Within this study, conceptualization of the project was carried out by [PERSON], [PERSON], and [PERSON]. Experimental methodology was planned by [PERSON] and [PERSON]. Formal analysis on data was performed by [PERSON] and [PERSON]. Investigation of study was carried out by [PERSON], [PERSON], and [PERSON] Resources were provided and created by [PERSON], [PERSON], and [PERSON]. Data curation and data management was the responsibility of [PERSON] and [PERSON]. For the manuscript, the original draft was prepared by [PERSON], [PERSON], [PERSON], and [PERSON]. Review and editing was performed by [PERSON], [PERSON], [PERSON], and [PERSON]. Data visualization was performed by, [PERSON], [PERSON], [PERSON], and [PERSON]. Supervision of work and project administration was the responsibility of [PERSON]. Internal funding was acquired by H.Z.Y.  
**Funding:** This research received no external funding. The APC was funded by the Department of materials science and engineering and College of Engineering at Virginia Tech.  
**Acknowledgments:** Part of this work is an outcome of a senior design project supported by the Department of Materials Science and Engineering at Virginia Tech, which the authors would like to acknowledge. The authors would also like to acknowledge [PERSON] and [PERSON] for their help in experiment.  
**Conflicts of Interest:** The authors declare no conflict of interest.