The repair of high strength, high performance 7075 aluminum alloy is essential for a broad range of aerospace and defense applications. However, it is challenging to implement it using traditional fusion welding-based approaches, owing to hot cracking and void formation during solidification. Here, the use of an emerging solid-state additive manufacturing technology, additive friction stir deposition, is explored for the repair of volume damages such as through -holes and grooves in 7075 aluminum alloy. Three repair experiments have been conducted: double through-hole filling, single through-hole filling, and long, wide-groove filling. In all experiments, additive friction stir deposition proves to be effective at filling the entire volume. Additionally, sufficient mixing between the deposited material and the side wall of the feature is always observed in the upper portions of the repair. Poor mixing and inadequate repair quality have been observed in deeper portions of the filling in some scenarios. Based on these observations, the advantages and disadvantages of using additive friction stir deposition for repairing volume damages are discussed. High quality and highly flexible repairs are expected with systematic optimization work on process control and repair strategy development in the future.

Metrics & MoreArticle Recommendations CONSPECTUS: Structural metal components play a vital role in a broad range of industries, from aerospace and automotive to infrastructure and defense. In service, these components can experience substantial wear, thermal fatigue, erosion, corrosion, or chemical reactions, resulting in significant surface or even volumetric damages. Replacement of these components is often energy-intensive and economically impractical. Structural repair, which aims to restore the original geometry while enabling good mechanical performance postrepair, can offset the costs dramatically. Depending on the additive capabilities and bonding mechanisms, structural repair technologies can be divided into four categories: nonadditive, nonmelting-based; nonadditive, melting-based; additive, nonmelting-based; additive, melting-based. Although melting-based approaches can be applied to various repair geometries with good precision, the underlying melting and solidification processes inevitably lead to crucial problems impacting the mechanical performance, such as solidification porosity, high residual stresses, dendritic microstructure formation, elemental segregation, hot cracking, and stress corrosion cracking. To fundamentally solve or minimize these problems, one may employ solid-state technologies that leverage ultrasonic vibration, friction stirring, or particle impact to facilitate metallurgical bond formation. For robust geometry restoration, an additive capability needs to be incorporated for continuous material feeding and precise deposition path control. Currently, two solid-state technologies satisfy the requirement, cold spray and additive friction stir deposition.In this Account, we discuss the structural repair enabled by solid-state metal additive manufacturing, focusing on (i) cold spray, which is a relatively established process, and (ii) additive friction stir deposition, which is an emerging process recently triggering significant research effortsthe authors are particularly invested in this process and are pioneering the research on process fundamentals and structural repair applications. In cold spray, a substrate is bombarded with small metal particles at high speed; upon impact, the particles and substrate co-deform, resulting in interfacial bonding and mechanical interlocking. In additive friction stir deposition, frictional heat is created after the rapidly rotating feed-rod contacts the substrate, followed by co-plastic deformation and mixing between the deposited material and substrate surface. This renders a strong interface with complex 3D features. Both cold spray and additive friction stir deposition can be applied to a wide range of repair geometries while preventing hot cracking and high thermal exposure. Although cold spray has better portability and spatial resolution than additive friction stir deposition, we believe that additive friction stir deposition is the top choice for repairing load-bearing components given its unparalleled capabilities of rendering equiaxed...