## 3 Results
### Long, Wide Groove Filling  
To fill the long, wide groove, the print head is first placed above the groove and begins to rotate at 400 RPM without traveling. Feed material is simultaneously forced down into the groove. The dynamic  
Figure 6: (**a**) Full cross-section of the single hole filling sample created by stitching together multiple optical micrographs. Highlighted regions show (**b**) the transition zone between the substrate plate and the deposited material, (**c**) the typical microstructure in the deposited material, and (**d**) a higher magnification view of the elongated grain region at the bottom of the repaired hole. contact between the feed material and the bottom of the groove leads to frictional heating so that the feed material is plasticized. Once the feed material begins spreading out under the print head to reach the side walls of the groove, slow traveling at a rate of 0.42 mm/s starts along the longitudinal direction of the groove with a material feed rate of 0.06 mm/s. Upon the completion of the first layer, the print head reverses direction to deposit another layer on top of the already deposited track. Finally, to ensure good filling, a third traverse is conducted over the two previous deposition layers, with the intention of pushing more material into the groove. Each layer after the first is approximately 0.76 mm in height. Figure 7a,b show the top view and cross-sectional view of the as-repaired groove, respectively. A stitched optical image for the entire cross-section of the repaired groove is shown in Figure 7c, which confirms complete volumetric filling, notwithstanding the increased volume to fill and the increased distance from the print head to the side wall of the 1/2\" wide groove. Unlike in single hole filling, no voids are observed after the groove filling. Moreover, the upper portions of the repair show sufficient mixing between the deposited material and the side wall of the groove, which is indicated by the lack of a distinct interface. Nevertheless, the depth of sufficient mixing (below the starting substrate surface level) is generally in the range of 1.31-1.68 mm, as highlighted by the two black arrows. This range is only about half of the groove depth. Addressing the inadequate mixing in the lower half of the repair is a critical necessity. Compared to the hole filling experiments where the feed rod diameter is too large to be placed into the hole, the groove filling experiment involves direct contact of the feed rod to the bottom of the wide groove. Even with such direct contact, the repair quality of the lower portions of the groove is still not satisfactory. This implies that the initial pass of the print head is insufficient at mixing the two materials and that subsequent passes are not able to penetrate deep enough to facilitate further mixing. Examining the interior region of the repaired groove, sharp internal boundary features are observed close to the bottom of the groove, which are implied by the stark black lines. These boundaries originate from the complex material flow occurring during repair as indicated by the flow lines, which can be  
Figure 7: **(a)** Top-down view of completed deposition for the groove fill sample with the tool traverse marked; **(b)** cross-sectional view of the groove fill sample taken at the red dotted-line; **(c)** full cross-section of the groove filling sample, created by stitching together several optical micrographs. controlled by the processing parameters and travel paths of additive friction stir deposition. A degree of material separation occurs in the boundary regions, making it highly susceptible to crack initiation and growth upon mechanical loading. Notably, the complete groove filling here is implemented by a single-track of deposition, even though the width of the groove is 33% larger than the width of the feed rod.