Grain refinement mechanism of ultrasonic application stage in wire arc additive manufacturing of alloy steel.

Ultrasound-assisted wire arc additive manufacturing (UA-WAAM) offers a promising route to refine the coarse columnar grains found in as-deposited components. However, the effects of ultrasound applied at different stages during the rapid solidification of the molten pool remain insufficiently understood, and the grain refinement mechanism requires clarification. A novel stage-specific ultrasonic application strategy was designed based on the phase transformation temperatures and thermal history of molten pool. Five discrete ultrasound intervention stages were investigated, before arc extinction, at arc extinction, during solidification, after solidification and continuous application. Microstructural evolution was characterized using optical microscopy and EBSD, while the cavitation dynamics and mechanical response of columnar grains were theoretically modeled to establish a quantitative fragmentation criterion. Results reveal that effective grain refinement occurs exclusively when ultrasound is applied during cooling and solidification stage, transforming coarse columnar grains into fine equiaxed and refined columnar grains. Ultrasound applied before arc extinction or after complete solidification produces negligible microstructural modification due to the absence of surviving nuclei or a liquid medium. Theoretical analysis reveals cavitation-induced grain fragmentation as a key mechanism, with predicted critical fragmentation lengths (15.3-25.8 μm) aligning quantitatively with the measured mean grain size (20.9 ± 3.7 μm). Acoustic streaming plays a supporting role by dispersing fragments as heterogeneous nucleation sites. The critical window for effective ultrasonic application is limited to the sub‑second interval from arc extinction to complete solidification. These findings establish a quantitative relationship between cavitation shock wave pressure and grain fragmentation behavior, providing a scientific foundation for optimizing energy-efficient ultrasonic strategies in WAAM.