## 2 Materials and Methods  
AA7075 is a commonly used material in the aerospace, automotive, and defense industries due to its high strength-to-weight ratio and corrosion-resistant properties [30; 31]. For this study, extruded AA7075-T6 rods were used as the feedstock material, and machined AA7075-T6 plates were used for the substrate material. AA7075 is an Al-Zn-Mg-Cu precipitate-strengthened alloy that derives its strength primarily from the formation of \(\eta\)' phases [32; 33]. The T6 temper generates relatively high strength due to the ideal generation of the Mg2 Zn and Al\({}_{2}\)CuMg strengthening precipitates [34; 35]. This alloy was chosen for the present study for its industry relevance and to allow for direct comparison to prior friction-stir processing studies [36; 37; 38]. Figure 1a shows a schematic of an AFSD build with geometry similar to depositions used in this experiment. A non-consumable hollow cylinder tool is rotated at high RPM (\(\omega\)) in tandem with a linear actuator applying a downward force, relative to the build direction, to the end of the consumable feed rod at the desired feed rate (f). As the tool traverses at a constant speed (v) over the substrate material, the feed rod material is deposited and mixed with the substrate. As shown in Figure 1b, during the second traverse in the longitudinal direction, a region of the recently deposited material is processed a second time in the overlap region. At the start of each layer, the tool dwells in place, generating the frictional heat and high shear forces required to achieve plastic flow. The tool dwells in the same position at each layer, located in the nearest corner of the build in Figure 1a, until the feedback force is measured by the linear actuator stabilizes, signifying the material has achieved plastic flow. Once the material has reached the necessary temperature, between 350\({}^{\circ}\) and 400\({}^{\circ}\) Celsius [17], and the feed rod is able to plasticly deform with relatively low force, and the tool begins traversing in the longitudinal direction (LD) and deposits the initial pass of material. The tool then moves orthogonally in the overlap direction (OD) until the center of the tool reaches the predetermined distance to create an overlap region of the desired size. The tool then travels parallel to the initial pass, but in the opposite direction, to deposit the return pass. During the return pass, a portion of the initial pass deposition is subjected to extreme plastic deformation a second time, mixing with the new material being deposited. The tool face then returns to the dwell zone before moving in the  
Figure 1: (**a**) Representative schematic of an in-progress parallel pass AFSD. (**b**) Section view with overlapping tool path schematic. Red and blue coloration is used to visually distinguish deposition layers. vertical build direction (BD) and begins the next layer in the same movement pattern. In this present study, the tool path is always followed in a clockwise pattern. During the AFSD build process, the rotation of the tool face causes non-uniform material flow. The advancing side (AS) is the side where the tangent line of the tool face's rotation is aligned in the same direction as the traversing direction of the tool's movement. The retreating side (RS) is the opposite side of the tool face, where the tangent line is parallel to the traverse direction, but opposite in magnitude. The tool face rotates in a counterclockwise direction throughout the entire deposition. This means that the center of the build, where material is overlapped, is always on the advancing side of the tool face in this present study. Figure 2a shows a model of the 38.1 mm diameter tool used in this study. This tool, made of H13 toll steel, has multiple 'teardrop' protrusions that generate additional shear force and friction in the substrate material. These protrusions allow the material to quickly approach the temperatures necessary for plastic flow and improves mixing within the material.