Fatigue life evaluation and cellular substructure role of laser powder bed fused 304L steel based on dissipative deformation mechanisms

This study presents an energy dissipation model based on the self-heating effect for rapidly evaluating high-cycle fatigue parameters of 304L stainless steel (SS) produced by laser powder bed fusion (LPBF). To obtain the two-dimensional temperature field evolution and verify the model accuracy, the conventional and stepped fatigue loading procedures were carried out with the in-situ monitoring of an infrared camera. Numerical simulations of the surface temperature fields during fatigue process were performed to investigate the effect of material geometry and material thermodynamics parameters (such as specific heat and thermal conductivity coefficient) on the self-heating effect. In addition, to explore the role of cellular substructures of LPBF 304L SS in energy dissipation and fatigue deformation mechanisms, the cell-free samples with no additional microstructural modifications were prepared by annealing process. The experimental results indicate that the model-predicted S-N curves are in good agreement with the experimental data. Under low-stress amplitudes, the entangled dislocations on the cellular substructures remained completely stable (without damped vibration) during cyclic deformation due to the pinning of misorientation, segregated atoms, and inclusions, thus not contributing to energy dissipation from anelastic mechanism. On the other hand, the cellular substructures improved the microstructural stability (such as high angle grain boundaries) by back stress strengthening, thus increasing the fatigue limit. This work contributes significant value to LPBF technology by providing a novel alternative method for fatigue life prediction and understanding the role of cellular substructure in energy dissipation and fatigue performance.