Grain refinement of Ti5321G alloy created by ultrasonic energy field during laser powder direct energy deposition

The mechanisms of grain refinement in Ti5321G alloy induced by an ultrasonic energy field (UEF) during laser powder direct energy deposition (LP-DED) were systematically investigated, focusing on the interplay between recrystallization in high-temperature solid deposition layers and ultrasonic cavitation-acoustic streaming effects during molten pool solidification. A novel experimental design was developed to decouple these mechanisms by creating four distinct UEF-action zones (N, S, L, S+L) within a single-pass multilayer sample. This approach enabled direct comparison of UEF’s dual effects: recrystallization in solidified layers and ultrasonic cavitation-acoustic streaming effects in liquid pools. The results demonstrated that UEF significantly refined microstructures, reducing average grain size by up to 64.1% (from 399.6 ± 28.6 μm to 143.1 ± 16.1 μm) in the S+L zone, while promoting columnar-to-equiaxed transition, with equiaxed grain probability increasing from 11.1% (without UEF) to 53.8%. Texture intensity was reduced by ~52.4%, and mechanical properties were enhanced, achieving a 6.2% increase in yield strength (702.0 ± 10.6 MPa) and 31.7% improvement in elongation. Crucially, this work reveals the synergistic effect of UEF’s dual-action mechanisms, where recrystallization and cavitation-acoustic streaming collectively enable nonlinear grain refinement. The study provides a groundbreaking strategy for microstructure control in additive manufacturing, eliminating the need complex post-processing, thereby advancing the industrial application of high-performance titanium components.