In-situ SEM investigation of fatigue crack propagation through cross-weld area in WAAM low-carbon steel and the role of microstructures in propagation behavior

In-situ SEM fatigue testing is performed to investigate crack propagation through the heat affected zone as well as in the base material and deposited material in a wire and arc additive manufacturing (WAAM) low-carbon steel part. The slip band formation and development prior to crack initiation and the crack growth rate are monitored in-situ and compared using fatigue testing specimens sampled from the base material, heat affected zone, WAAM zone, and cross-weld zone. Results show that the cross-weld zone specimen has the lowest crack growth rate, followed by the heat affected zone specimen and the two base material specimens. The effect of banded pearlite, acicular ferrite, and grain boundary and orientation is discussed with the aid of optical metallographic images, SEM, and EBSD maps. The mechanisms of the lowest rate observed in the cross-weld zone specimen are the pearlite bands preventing slip transfer, the high angle grain boundary of ultra-fine acicular ferrite hindering the crack propagation, as well as the basketweave structure promoting a zigzag growth path.