Abstract: Wire and arc additive manufacturing (WAAM) has garnered significant attention in aerospace, transportation, and other fields as an efficient and low-cost remanufacturing technology for metallic components. Unlike traditional welding processes, the heat-affected zone (HAZ) in WAAM remanufacturing exhibits a more complex microstructure due to repeated thermal cycles and heat accumulation during multi-layer and multi-pass deposition. This study investigates the fatigue crack growth behavior in the HAZ of low-carbon steel components fabricated by WAAM remanufacturing. In-situ fatigue tests were conducted on specimens extracted from different locations of the remanufactured part to examine the crack growth behavior. The influence of microstructure on fatigue crack growth was discussed. Three types of specimens were designed: HAZ specimens, WAAM-deposited metal specimens, and hybrid specimens containing the HAZ, WAAM-deposited zone, and base material. Furthermore, an in-situ fatigue testing platform synchronized with acoustic emission (AE) monitoring was established to simultaneously capture microstructural evolution and AE signals released during crack propagation. Results indicate that the hybrid specimens exhibited significant crack path deflection and an overall lower crack growth rate compared to the HAZ and WAAM-deposited metal specimens, demonstrating superior fatigue resistance. Microstructural analysis revealed that the hybrid zone possessed a high proportion of high-angle grain boundaries (HAGBs) and clusters of acicular ferrite, which significantly hinder the continuous propagation of slip bands and the development of plastic zones, resulting in a zigzag crack propagation path. Based on in-situ AE measurements, this study proposes a novel nonlinear parameter derived from sideband count analysis to characterize the energy released during crack deflection events at grain boundaries with varying misorientation angles. This approach provides a new perspective for in-situ material characterization using AE signals.

Key words: heat-affected zone, in-situ fatigue test, acoustic emission, nonlinear parameter, crack deflection angle