Numerical analysis of powder-induced gas porosity in directed energy deposition: Formation, evolution, and mitigation

Directed energy deposition (DED) is an important branch of metal additive manufacturing and holds significant potential for the manufacturing and in-situ repairing of complex parts. However, the gas porosity defect is inevitably introduced in the DEDed parts, which severely affects the fatigue performance of the parts. This paper innovatively combines the discrete element method (DEM) and computational fluid dynamics (CFD) model to trace the transformation from powder dynamics to bubble evolution in the molten pool and finally to pore entrapment in solidified tracks. The results show that the fluid drag force play the dominate role on bubble evolution in the molten pool. Specifically, in the straight flow region, small bubbles escape directly from the molten pool, while in the vortex region, their satellite-like rotational motions promote coalescence and entrapment, leading to higher porosity density at the top and bottom of the track. Finally, two methods, namely powder stream size tailoring and laser beam shaping are proposed to mitigate gas porosity. This work can further enhance the understanding of coupled physical phenomena and gas porosity formation in DED.