## 3 Results  
### Double Through-Hole Filling  
For double hole filling, the deposition begins close to the edge of the large hole (i.e., the 1/4\" hole), and then the print head travels toward the hole while simultaneously depositing a continuous track of AA 7075 on the top surface of the AA 7075 plate. The in-plane motion of the print head stops when it aligns with the center of the hole. This is followed by a dwelling phase, in which the print head continues to rotate and deposit new material without in-plane motion, in order to allow for enough AA 7075 to fill the hole. For this experiment, the tool rotation rate is held constant at a rate of 400 RPM, the material feed rate is 0.1 mm/s, and the in-plane velocity is 0.32 mm/s when traveling. Figure 2 shows several snapshots of the traveling and dwelling phases of this repair approach. The print head is held stationary until the edge of the deposition track begins to push out beyond the edges of the tool print head, suggesting that the hole is fully filled and there is an excess of material. Once the excess of material is observed, in-plane motion towards the second hole (i.e., the 1/8\" hole) begins. The in-plane motion stops again when the print head aligns with the center of the second hole, which is also followed by a dwelling step for material filling. The entire repair process is illustrated in Figure 3a. \begin{table}
\begin{tabular}{c c  
Figure 3b-d show the AA 7075 plate after repair using additive friction stir deposition. Figure 3b shows the bottom of the plate after repair. It can be seen that the deposited material is able to reach the bottom of the cylindrical holes. Figure 3c,d show that the two holes are completely filled by the deposited AA 7075 from the top to the bottom, without apparent defects. The side wall of the hole appears to be fully mixed with the deposited material, manifesting indiscernible interfaces in the cross-sectional images. Figure 3: **(a)** Schematic of the double hole filling experiment; **(b)** bottom view of the filled holes; **(c)** cross-sectional views of the filled holes (holes cut in half); **(d)** underside of the cross-sectional view of the filled holes. Figure 2: Depiction of hole filling by additive friction stir deposition, wherein the tool travels toward the hole (highlighted in red in image **(a)**) and then dwells. Snapshots **(a)** right before hole filling; **(b)** start of hole filling; **(c)** partial covering of the hole during filling; **(d)** complete coverage of the hole during filling; **(e)** dwelling over the hole during filling; **(f)** tool retraction. We have also examined the microstructure of the repaired holes. Figure 4a shows a large-area SEM micrograph of the lower half of the 1/4\" hole after repair, which again confirms that the deposited material fills the entire cylindrical through-hole. In addition, there is no defect observed at the interface (marked as 'hole wall edges') between the deposited material and the surface of the hole, suggesting that there is sufficient mixing and adhesion between the two. A zoomed-in image of the interfacial region (after electro-etching) is shown in Figure 4b, featuring a curved, rather than a straight and sharp interface. The curved interface is a sign of co-plastic deformation of the deposited material and the side wall material, which involves intensive material flow and mixing between the two. Such curved interfaces and the associated structural interlocking are expected to lead to substantially stronger interfacial bonding than alternative coating or repair processes. More importantly, Figure 4b shows distinct microstructures between the filled AA 7075 and AA 7075 plate. The former generally exhibits an equiaxed and refined microstructure, with the average grain size of \(3.4\pm 0.7\) um, whereas the latter exhibits elongated grains that are typical of the rolling microstructure of Al alloys. It is noted that the starting microstructure of the feed material is the same as the plate--i.e., with elongated grains.