Heat and mass transfer in electron beam additive manufacturing

The electron beam wire-feed additive manufacturing (EBAM) technology utilizes a high-energy electron beam as a heat source to bombard a metal wire in the vacuum environment to form a molten pool. The stability and quality of the deposition mainly depend on the transition behavior of the droplet. To conduct this research, a high-fidelity thermo-fluid dynamic simulation model based on the local reconstruction of the 3D model was developed and verified. The model used the Monte-Carlo algorithm to update the scattering trajectories and residual energies of high-energy electrons on the irradiated surface of the wire and molten pool at each time step in the numerical calculation. Quantitatively analyzed the "electron-wire" instantaneous energy coupling mechanism in the droplet transition mode, insertion transition mode, and liquid bridge transition mode, and finally established an energy-matching criterion for maintaining a stable deposited track morphology. The results show that during the initial stage of wire melting, the droplet will climb along the wire feeding direction, grow under gravity, and eventually drop under the combined effects of gravity and recoil pressure. Different forms of molten droplets will form various solidification morphology, with the liquid bridge transition mode being the most desirable. To maintain a stable liquid bridge transition, it is necessary to control the droplet generation height within the range of 0-2.7 mm. The collapse of the deposited track, caused by heat accumulation during the multilayer deposition, would also alter the droplet generation height. Therefore, to maintain the liquid bridge transition mode in the multilayer multi-channel forming process, it is also necessary to ensure that the energy density of the deposited track is slightly greater than 1.16x1010 J/m3 and the energy density of the wire is slightly greater than 2.63x1010 J/m3.