## 2 Materials and Methods  
All additive friction stir depositions were performed using a MELD R2 system (MELD Manufacturing Corporation, Christiansburg, VA, USA) with a rotation rate of 400 RPM. Three types of experiments were performed. Experiment 1 involved double hole filling, in which two cylindrical through-holes with the diameters of 6.35 mm (i.e., 1/4\({}^{\prime\prime}\)) and 3.125 mm (i.e., 1/8\({}^{\prime\prime}\)) were drilled in series into an AA 7075 plate. In this work, double hole filling using additive friction stir deposition was implemented by (i) traveling of the tool (i.e., the print head); (ii) dwelling (i.e., zero traverse speed); and (iii) traveling again. Experiment 2 involved single hole filling, in which one cylindrical through-hole with the diameter of 6.35 mm (i.e., 1/4\") was drilled into the plate. In this work, single hole filling using additive friction stir deposition was implemented by (i) traveling; (ii) dwelling; and (iii) the lift of the tool. In Experiment 1 and Experiment 2, the traverse speed during tool traveling was 0.32 mm/s and the linear feed rate was 0.1 mm/s. Experiment 3 involved the filling of long, wide grooves, which were 12.7 mm (i.e., 1/2\") wide, 3.175 mm (i.e., 1/8\") deep, and 304.8 mm (i.e., 12\") long. In this work, such groove filling was implemented by the slow traveling of the tool without dwelling, wherein the traverse speed was 0.42 mm/s and the linear feed rate was 0.06 mm/s. The details of the three types of experiments are summarized in Table 1. Figure 1 shows an image of the additive friction stir deposition facility along with the hole filling and groove filling illustrations.  
Figure 1: (**a**) Example of the additive friction stir deposition process; (**b**) schematic of the hole filling conducted; (**c**) schematic of the groove filling method conducted.  
In all experiments, 6.35 mm thick commercial AA 7075-T651 plates were used as the substrate, with the cylindrical holes and long wide grooves machined as specified in Table 1. The measured composition of the plates is summarized in Table 2. The feed material for additive friction stir deposition was also cut from the commercial Al 7075-T651 plates, with a cross-section of 9.535 \(\times\) 9.535 mm\({}^{2}\). After the repair experiments, cross-sections of the filled holes and grooves were cut and polished using standard metallographic techniques. For electron microscopy, the samples were first etched with a caustic mixture, consisting of 1 g of NaOH and 100 mL of water, in order to reveal the grain boundaries. For optical microscopy, samples were electro-etched using 50 mL of [PERSON]'s reagent etching solution--comprised of 1.9 mL of 48 wt.% HBF\({}_{4}\) solution and 48.7 mL of H\({}_{2}\)O--with an etching voltage of 30 V for 1 min. Polarized optical microscopy (Zeiss, Oberkochen, Germany) and scanning electron microscopy (SEM; Helios Nanolab 600 DualBeam, Waltham, MA, USA) were used to characterize the microstructures after repair. For optical microscopy, multiple images were taken and stitched together using ImageJ, an open-source image processing software, to allow for a holistic view of the entire cross-section. Microhardness testing was performed using a Leco LM100 AT (St. Joseph, MI, USA) microhardness tester at 300 N load and with 10 s dwell time. Hardness for the single hole filled sample was measured using 20 points along the width of the repair cross-section with a spacing of 0.5 mm and 14 points along the height of the repair with a spacing of 0.4 mm. The groove sample was tested at 20 points along the width of the repair with a spacing of 1.0 mm and 9 points along the height of the repair with a spacing of 0.6 mm.