• Ultrasonic energy field (UEF) reduced the average grain size of Ti5321G alloy by 64.2% (from 399.6 μm to 143.1 μm), promoting a columnar-to-equiaxed transition.
• The synergistic dual-action mechanism of UEF—recrystallization in solid layers and cavitation-acoustic streaming in the molten pool—enables highly effective grain refinement.
• Mechanical properties were significantly enhanced: yield strength increased by 6.2% and elongation improved by 31.7%.
• The novel experimental design decouples UEF mechanisms, offering a practical strategy for microstructural control in additive manufacturing without complex post-processing.