High-cycle fatigue behaviors of twinning-induced plasticity steels fabricated by laser powder bed fusion

Twinning-induced plasticity (TWIP) steels fabricated by laser powder bed fusion (LPBF) were subjected to heat treatments at 400°C for 1 h and 800°C for 30 min, followed by high-cycle fatigue (HCF) testing. The results reveal that, despite the high static strength achieved in LPBF TWIP steels, their fatigue strength remains relatively low. The heat treatments improved fatigue resistance by relieving residual stresses and/or disrupting cellular structures, particularly under low stress amplitudes. A predictive H-parameter model was proposed by integrating defect size, location, circularity, along with a weighting factor (h), to evaluate fatigue life under various conditions. In the as-built condition, decreasing stress amplitude caused a transition in crack initiation modes from surface lack-of-fusion (LOF) defects to internal {111} slip planes governed by microstructural features. The latter involves the activation of multiple slip systems with maximum Schmid factors exceeding 0.4 along the crack path, driven by the inhibition of dislocation motion by cellular structures. This promotes strong interactions among nano-twins and stacking faults (SFs) within multiple active slip systems, leading to localized stress concentrations at these intersections and twin boundaries. In addition, both low-angle grain boundaries (LAGBs) and cellular structures may hinder intragranular crack propagation and contribute to crack deflection.