Effects of ultrasound on multilayer forming mechanism of Inconel 718 in directed energy deposition

Advanced 3D metal printing is getting popularity recently while the complexities during the forming are not totally clear. In particular, research on the interaction mechanism of molten pool–powder flow–spatter in ultrasonic vibration-assisted directed energy deposition (UV-A DED) is limited. In this study, a kinematic model of powder flow is established based on gas–solid particle two-phase fluid mechanics. Combined with the experiments, the optimal stand-off distance is determined between the powder nozzle outlet and substrate. In addition, the effect of ultrasound on molten pool–spatter–powder flow in the cladding process is explored through the high-speed imaging visualisation. Results show that the evolution of cavitation bubbles affects the Marangoni flow in the molten pool and causes the lamination phenomenon in the cladding layer. The microstructure and microhardness of the formed workpieces obtained under different scanning strategies are discussed as important indices to evaluate the performance of a workpiece. The study demonstrates a new forming strategy combining UV-A DED with traditional DED which not only reduces defects caused by lamination effectively, but also refines the microstructure and improves the microhardness. ● Based on the gas–solid particle two-phase fluid mechanics, a kinematic model of powder flow is established. ● The molten pool evolution is analysed by high-speed imaging visualization and the lamination is observed in UV-A DED. ● Different scanning strategies (DED, UV-A DED, the alternate strategy) are compared concerning the forming quality.