Anomalous fatigue crack propagation behavior in near-threshold region of L-PBF prepared austenitic stainless steel

Laser powder bed fusion (L-PBF) process produces a specific non-equilibrium microstructure with properties significantly different from those of conventionally processed materials. In the present study, the near-threshold fatigue crack propagation in austenitic stainless steel 304L processed by L-PBF was investigated. Three series of specimens with different orientation of initial notch with respect to build direction were manufactured in order to evaluate the effect of specimen orientation on the near-threshold fatigue crack propagation behavior. The results showed absence of the orientation dependence of the fatigue crack propagation behavior. In addition, abnormally low threshold stress intensity factor values were recorded, which were attributed to the absence of crack closure even at low load ratio (R = 0.1). In order to explain observed behavior, the specimens of selected orientation were heat treated to relieve build-in residual stresses (HT1) and to create recrystallized micro-structure (HT2) comparable to conventionally processed (wrought) stainless steels. It was found that the char-acteristic microstructure produced by L-PBF is the main reason for the absence of crack closure and the low threshold values at low load ratios. As-built microstructure containing sub-micron dislocation cell-substructure is prone to cyclic instability. In the crack tip region, cyclic plasticity results in strain-induced phase transformation and continuous thin martensitic layer is formed in the crack vicinity. The induced martensite phase is softer compared to the austenite matrix strengthened by cell-substructure. Together with cyclic instability of the ma-trix, the macroscopic cyclic softening occurs as the result within the crack tip region. Under such conditions, the formation of the plasticity-induced and roughness-induced crack closure is significantly reduced and macro-scopic resistance to the fatigue crack propagation is low.