Evolution and elimination of pore in molten pool during directed energy deposition assisted by ultrasonic vibration

[Display omitted] •Induced the acoustic streaming to accelerate gas bubbles to escape the molten pool.•Drag force causes the significant differences in the motion trajectories of bubbles under acoustic streaming.•Using acoustic streaming to concentrate non-escape bubbles at the molten pool edge.•The hybrid processing methods has enhanced the porosity reduction effect by ultrasonic effect. Pore defect plays an important factor on leading to a decrease in the strength performance and reliability of deposition layers manufactured by directed energy deposition (DED). The ultrasonic energy field assistant method usually improves the structure of materials, now is induced to reduce porosity by acoustic streaming driven effect in directed energy deposition process. In this work, 1Cr12Ni3MoVN powder (29% hollow particles) was used to controllable generate bubbles in the molten pool to induce the imperfect deposition conditions in the experiments. The distribution of pores with different laser processing parameters under ultrasonic vibration (UV) was investigated, and the principle that ultrasonic vibration drives the bubble motion under the complex physical environment was also analyzed by numerical simulation. The results show that acoustic streaming lead by UV effect, in competition with other flows (Marangoni flow and non-isothermal flow), makes the small bubbles to escape or large bubbles to move toward the edge of the molten pool. In general, UV reduces porosity from an average of 0.61% to 0.31%. Combined with the subtractive method, the porosity reduction rate has been improved from 45% to 82%. UV finally resulted in an average increase of 17% in the elongation of the material. This work provides the understanding on the driving effect of acoustic streaming on bubble motion and guide a novel way to reduce the porosity of DED technology.