## 3 Results
### Single Through-Hole Filling  
For single hole filling, the deposition starts close to the edge of the hole. This is followed by the print head traveling along the centerline of the hole until it reaches the center of the hole. After that, the print head stays over the hole for a pre-determined amount of time: the dwell time of 3 min, which controls the amount of filling material. Once the dwell phase finishes, no further traveling is applied. Instead, the print head is lifted while still rotating, so that the feed rod breaks off from the deposited material. Similarly to Experiment 1, 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. Figure 5 shows the top and cross-sectional views of the as-repaired sample with the 3 min dwell time. As can be seen from the images, additive friction stir deposition does not result in keyholes that are observed in friction stir welding; the breakage of the feed rod merely results in a rough surface at the end of deposition. In practical repair applications, the tool head could be retracted more slowly to allow this area to be flatter. A representative microstructure image of the repaired hole in cross-sectional view is shown in Figure 6a, which is comprised of multiple optical micrographs stitched together. Magnified views of various features in 6a are illustrated in Figure 6b-d. As a whole, Figure 6a depicts the extent and shape of the mixing that occurs between the deposited material and the plate. The majority of the cylindrical through-hole has been successfully filled by the deposited AA 7075. Additionally, sufficient mixing between the deposited material and side wall of the hole is observed in the upper portions of the repair. These good mixing zones are generally comprised of a gradual transition from the elongated grains of the AA 7075 plate to the fine, equiaxed grains of the deposited AA 7075, meaning that no distinct interface is present. An example of such a transition zone is illustrated in Figure 6b, which also shows varying degrees of grain deformation and grain refinement within this zone. The typical microstructural features of the refined equiaxed grains (grain size \(<10\) um) of the deposited AA 7075 are shown in Figure 6c. It should be noted that the black spots that appear in Figure 6b,c are most likely due to pitting, which is a result of over-etching during sample preparation. Figure 5: (**a**) Top-down view of the completed deposition for the hole fill sample with the tool traverse marked; (**b**) cross-sectional view of hole fill sample taken at the red dotted-line. The repair quality of this sample is visibly worse when compared to the double hole fill sample in Section 3.1. In the lower portions of the repaired hole shown at the bottom of Figure 6a, a void of \(\sim\)1 mm x 1 mm in cross-section is observed; moreover, there are distinct, sharp interfaces separating the side wall of the hole and the deposited material (Figure 6d), suggesting inadequate mixing and weak interfacial adhesion. The starting points of these distinct interfaces are highlighted by the two black arrows in Figure 6a, showing that only the top \(\sim\)1.6-3.3 mm portion of the hole has been well mixed. The depth of the sufficient mixing zone is greater on the left side where the print head first travels over the hole. Across the sharp interface, the inside features fine grains from the deposition, whereas the outside features the elongated grains that are characteristic of the rolled plate. Intriguingly, Figure 6a depicts a large section of elongated grains present at the bottom of the hole underneath the deposited material. Note that these elongated grains are only on the side of the hole from which the print head passes over, i.e., the left side of the image. Therefore, this unique region possibly originates from the plate material near the hole. A mechanism is proposed to explain this finding in Section 4.1.