Ultrafine grain control by ultrasonic vibrations in directed energy deposition

Microstructural coarsening and heterogeneity are unavoidable problems in additive manufacturing (AM). To realize ultrafine grain control by ultrasonic vibrations in DED, we designed ultrasonic-assisted directed energy deposition (UADED) experiment and established an ultrasonic-heat coupling model. Based on the established patterns, experiments were designed to fabricate AM samples under ultrasonic vibrations, and ultrafine grains were obtained. Grain morphology was regulated by ultrasonic vibrations. Compared with traditional DED, the average grain size is decreased by 52.9 % and the area proportion of equiaxed grain is increased by 41.2 % in case of 30 μm of vibration amplitude, 20 kHz of vibration frequency, 6 mm/s of scanning speed, and 800 W of laser power. Combined with experimental observations, the coupling model provides the process window for the microstructural control, with the computational accuracy reaching 96.8 %. The workpiece temperature is reduced by 230 K and the nucleation rate is increased by 36.3 % under ultrasonic excitation. The model illustrates that UA enhances heterogeneous nucleation by improving melt pool flow, providing a novel approach for microstructure control.