Exploring the strengthening mechanisms of additive manufactured metals treated by ultrasonic nanocrystal surface modification

This study investigates the use of ultrasonic nanocrystal surface modification (UNSM) to enhance the surface integrity and mechanical properties of stainless steel fabricated using selective laser melting (SLM). The improved yield strength is primarily obtained from grain refinement, deformation-induced martensitic, and high density of dislocations and deformation twins. The superior fatigue resistance is attributed to the synergistic effect of a reduction in surface and subsurface defects, dense dislocation substructure, high-strength martensitic phase, and a high amplitude of compressive residual stress (CRS) within the gradient deformation layer, collectively suppressing crack initiation. Microstructure evolution and CRS relaxation behavior of UNSM-treated samples during cyclic loading were examined. The results revealed that the benefits of CRS are relatively constrained due to its rapid relaxation under cyclic loading at a moderate stress level (550 MPa). In contrast, the gradient nanostructure remained stable under cyclic loading at the same stress level, exhibiting limited plastic deformation and grain coarsening. This indicates that the gradient nanostructure plays a more significant role than the CRS in delaying fatigue crack initiation and propagation at high stress levels. These findings provide valuable insights for identifying the dominant factor responsible for the improvement in fatigue resistance of SLM components after surface-strengthening treatment.