Solid-State Metal Additive Manufacturing for Structural Repair

Abstract

ConspectusStructural 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 microstructures and wrought-like mechanical properties. Regarding niche repair applications, cold spray is particularly suited for field repair of surface damage; our previous work has shown great promise of using additive friction stir deposition for underwater and large component repair. For future research in cold spray, strategies are needed to eliminate porosity and improve the as-repaired mechanical properties, especially when depositing high strength-to-weight ratio materials. For additive friction stir deposition, we hope to improve the spatial resolution and portability, possibly by down-scaling, and to enable robust repair of components made of high-temperature, high-strength materials.