Impact of Defects on Interfacial Bonding in Multilayer Friction Surfacing Deposited Metal Additive Structure

Abstract Additive friction surfacing (AFS) deposition is a process used for developing large structural products, where multilayer deposition is inherently required. Even using optimized parameters, fundamental process limitations lead to the formation of nonbonded edges and interfacial defects in multilayer structures. However, product performance grossly depends on the presence of defects and, more crucially, their impact on interfacial bonding. Edge trimming and intermediate surface machining after each layer deposition can remove edge defects and interfacial defects to a certain extent. Thus, machining cannot be a viable solution for the complete removal of internal defects. This article investigates the impact of defects in multilayer AFS-deposited structures by analyzing X-ray microcomputed tomography (μ-CT) images and correlating them with flexural and interlayer bonding properties. Detailed defect analysis revealed that stress concentration occurred at sharp edges, and their subsequent propagation significantly weakened interfacial bonding between layers. Additionally, insights into the deposition and defect formation mechanisms enabled effective mitigation of defects through friction stir processing (FSP). With the reduced defect volume and improved sphericity of residual defects achieved through the FSP operation, the fabricated structures demonstrated 43.93% higher flexural strength and no interlayer delamination under extreme bending.