Fatigue crack propagation in functionally graded bi-material steel obtained through wire-arc additive manufacturing

• Fatigue crack behaviour of HSLA-AS steel bi-material WAAM parts is elucidated. • Paris parameters are obtained for HSLA and AS steel, and the bi-material interface. • Metastable austenite at the interface decelerates crack propagation by a TRIP effect. • Opposingly, the presence of allotropic martensite promotes crack propagation. • The interface geometry leads to anisotropy in fatigue crack growth rates. This study concentrates on the fatigue crack propagation behaviour of a high-strength low-alloy (HSLA) steel and austenitic stainless (AS) steel bi-material part, as obtained by wire arc additive manufacturing (WAAM). Due to partial mixing in the weld pool, the first layer of AS steel laid onto the previously deposited HSLA steel results in a diluted interface layer of distinct chemical and microstructural characteristics. Average Paris parameters are obtained for the interface layer along transverse and longitudinal planes to the deposition direction (BD-LD plane: m = 2.79, log 10 (C) = –7.83 log 10 (da/dN)) (BD-TD plane: m = 3.47, log 10 (C) = –8.39 log 10 (da/dN)). However, it is observed that this interface layer manifests an intriguing crack propagation behaviour. FCGR consistently drop as the crack front transitions from undiluted AS steel to the interface. At ΔK = 20 MPa⋅m 0.5 , the greatest Δ is −0.77 log 10 steps (R = 0.1). As cracks near the HSLA fusion line, rates re-accelerate up to + 0.75 log 10 steps (R = 0.5). The phenomenon is attributed to the interplay between deformation-induced martensitic transformation and pre-existing allotropic martensite. Our findings, derived from a series of fatigue tests in correlation with multiscale microstructural and fracture characterization, offer insights into the damage-tolerant behaviour of these bi-material structures.