In situ thermal monitoring optimizes AFSD preheating: Feed strategy controls heat transfer, microstructure, and mechanical properties

This study reports the first application of a self-developed in-situ monitoring device to record interfacial thermal profiles during the preheating stage of additive friction stir deposition (AFSD). By combining finite element modeling with experimental validation, the work establishes a quantitative link between feed length, heat generation, and thermal distribution. Based on this understanding, three preheating strategies were designed and compared. Results show that the preheating process evolves through three characteristic stages driven by interfacial friction, and that heat output increases linearly with feed length. Among the evaluated strategies, the stepped growth (SG) approach promoted effective recrystallization and dislocation strengthening, leading to higher hardness and strength while reducing variations between preheating and steady-state regions. These findings provide mechanistic insight into AFSD preheating and suggest a practical route to improve the consistency of multilayer additive manufacturing.