## 1 Introduction  
High strength aluminum alloy components are widely used in aircraft, sporting equipment, and other safety-critical applications [1; 2]. With corrosion, wear, or impact damages, these components need to either be replaced--which is expensive, time consuming, and inefficient--or repaired [3]. For repair, fusion welding has proved highly successful for a broad range of metals. However, using fusion welding to repair aluminum alloy (AA) 7075 components is extremely challenging owing to the material's poor weldability, which is characterized by hot cracking and void formation during solidification and susceptibility to stress corrosion cracking after welding [4; 5]. The high failure probability and unpredictable failure time from fusion welding have incited numerous studies on solid-state repair for AA 7075 [6; 7; 8; 9]. Friction stir-enabled processes are among the most promising repair approaches for AA 7075, as the solid-state nature avoids hot cracking and high residual stresses while the intensive material flow promotes strong bonding. For example, friction stir welding and friction stir processing can heal surface cracks along the tool path, courtesy of the plastic material flow around the cracks driven by the shoulder rotation and traverse motion [10]. One drawback of these techniques is that they only reallocate the existing material. Without material addition, these friction stir-related processes are unable to heal wide cracks or repair volume defects [11; 12]. Another drawback is the keyhole formed as the rotating tool retracts from the welded plate, which leaves an additional volume defect to be repaired. These drawbacks can be addressed by equipping the friction stir-related processes with filler materials. For example, in friction taper plug welding, a tapered plug is co-axially forced into a keyhole with a similar taper. This method has proved successful at sealing through-holes left by the friction stir welding of 10 mm-thick AA 2219-T87 plates [13]. Here, the taper angles of the hole and plug should be compatible, otherwise defects are likely to form at the lower part of the weld. As a result, this technique performs better for tapered holes than standard cylindrical through-holes [14; 15]. Derived from friction taper plug welding, filling friction stir welding enables better repairs of volume defects, by adding a shoulder portion on the tapered plug to avoid stress concentration at the plug-hole interface. This technique has proven effective at sealing the exit holes in similar Al alloy repair (e.g., with an Al-Cu-Mg plug and an Al-Cu-Mg plate) [16], as well as in dissimilar Al alloy repair (e.g., with an AA 7075 plug and an AA 2219 plate) [17]. Other applications of a similar approach include vertical compensation friction stir welding, friction bit joining, and stationary shoulder friction stir welding [17; 18; 19; 20; 21; 22]. Recently, refill friction stir spot welding has been demonstrated to be effective at filling keyholes in AA 6061-T6 [11] as well as AA 7075-T651 [23], with the welded samples showing ultimate tensile strength as high as 74% of the base material. While the 'friction stir-filler' solutions have been effective at filling volume defects like keyholes, they are stationary and require an additional step to place the filler plug of desired geometry into the hole before repair. Moreover, without the capacity to continuously add material, these processes are limited to certain defect geometries defined by the size and shape of the hole. In order to repair various types of volume damages in AA 7075--including keyholes, long grooves, large-scale corrosion or wear damages--it is vital to develop a versatile solid-state technique that can continuously supply filler material, while also allowing for digital control of the material addition and repair paths. Such a technique remained elusive until the recent emergence of a solid-state metal additive manufacturing technique--additive friction stir deposition [24; 25; 26].