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The damage to fastener holes in aerospace aluminum structures presents significant challenges for aircraft durability, and conventional bushing methods for repairing oversized holes often fall short due to the lack of metallurgical bonding and limited edge distance availability. This study investigates additive friction stir deposition, a non-melting additive process, as a viable alternative for the structural repair of aerospace fastener holes. The repair process, demonstrated on AA7050 (Al-Zn-Mg-Cu-Zr) hole structures, involves filling oversized holes with new material and machining to restore the original hole size. The repaired hole coupons are defect-free and exhibit good fatigue performance under fully reversed tension-compression loading (R = -1). At a nominal stress amplitude of 123.5 MPa, the average number of cycles to failure is 12,666 for unrepaired baseline coupons and 17,372 for effectively repaired coupons. Restoring complex geometries without compromising fatigue performance has been difficult in aerospace applications; this study marks the first demonstration of additive repair that consistently outperforms the unrepaired baseline coupons. Notably, the result is achieved through a low-energy, cost-effective solution without the need for post-repair heat treatment. Except for a few outliers, the post-repair fatigue performance generally remains inferior to that of undamaged, pristine coupons, likely due to precipitate evolution in AA7050 caused by the thermomechanical processing nature of additive friction stir deposition. This evolution weakens the repair region and the adjacent base material, leading to faster crack initiation and growth compared to the properly aged base material, AA7050-T7451.

This letter examines the potential and challenges of using additive friction stir deposition for large-scale near-net-shaping of 7xxx Al alloys, particularly AA7075 and AA7050. Compared to fusion-based additive manufacturing, which is unable to print these alloys without feedstock modification or composition change, additive friction stir deposition is an emerging solid-state additive process with a thermomechanical processing nature that can render fully-dense AA7075 and AA7050 in the as-deposited state. More importantly, wrought-like tensile properties have been demonstrated in the deposited AA7075 based on the original composition. The process is well-suited for large-scale printing due to its high build rate and scalability. Significant challenges nevertheless exist in process and post-process control, in addition to the impurity from lubrication.

Additive friction stir deposition (AFSD), a deformation-based, near-net-shaping additive technology, is used to consolidate Al chips from automotive castings to produce fully-dense components, while addressing the energy, environmental, and efficiency challenges associated with recycling these chips via melting. Cold pressing of the chips results in feed-rods with a relative density of 68%. AFSD of these porous feed-rods leads to porosity-free material in the as-printed state. Compared to the base material of bulk cast Al, the as-printed material increases the tensile elongation from less than 1% to 17.8%, while exhibiting significant strain hardening. This upcycling effect is shown to originate from microstructure evolution during deposition, including the second-phase particles and the grain structure. The received Al chips have a hypereutectic composition and thus contain a high proportion of primary Si particles. The minor presence of Fe causes additional intermetallic particles as well. After deposition, these Si and Fe-based particles are refined, spherodized, and uniformly distributed in the Al matrix. Meanwhile, the cast Al microstructure is converted to an equiaxed grain structure with the grain size reduced from ∼ 25 μm to ∼ 2 μm. Finally, the energy consumption of AFSD-enabled upcycling is compared to melt-based recycling; potential pathways are evaluated for energy consumption reduction.

Given the challenges in feed material supply and quality control, metal additive manufacturing has rarely been implemented in austere environments, especially underwater. This paper explores the underwater operation potential of an emerging solid-state additive technology: additive friction stir deposition, wherein material feeding and bonding are enabled by mechanical forces with minimal influences from water. It is demonstrated that additive friction stir deposition of 304 stainless steel can be successfully performed with the print head and substrate immersed in water. High temperature is reached in the deposition zone (>60% melting temperature); the material deposition behavior is similar to that in typical open-air operation. The as-deposited material is fully-dense, having fewer annealing twins and a substantially smaller grain size than the feed material (4.98 μm vs. 31.44 μm). Such microstructural changes stem from dynamic recrystallization caused by the large strain and high temperature introduced during deposition. In addition to grain refinement, small equiaxed dispersoids (∼2–3 μm or less) are formed and evenly distributed in the austenite steel matrix. Rich in Cr, Mn, and O, these particles likely result from the reaction between the elements in stainless steel and water at elevated temperatures.