In electron beam three-dimensional (3D) printing, the metal-transfer behaviors play significant roles in determining the quality of the end product; however, these mechanisms have not yet been well understood. In the present study, we performed plenty of experiments and novel modeling of the wire feeding based electron beam 3D printing to reveal the metal-transfer mechanisms in depth. The coupling behaviors of the heat transfer and the fluid flow in the molten pool as a function of the process parameters in the electron beam 3D printing of the Ti-6Al-4 V alloys were simulated. The simulation results reasonably agree with the experimental data. Three types of metal-transfer modes (liquid bridge transition, droplet transition, and intermediate transition) are reproduced by simulations and confirmed by the experiments. The results show that as the heat input increases, the transfer mode changes from the droplet transfer mode to the liquid bridge mode. Therein, the liquid bridge transfer is the best for ensuring good print quality because of the stable metal-transfer behavior. More accurately, the liquid bridge is a dynamic equilibrium process. In the process, the metal transfer is mainly driven by the recoil pressure, while the surface tension always tends to break the bridge. The interaction between the two factors leads to the oscillation of the liquid bridge geometry morphology in the forming process, where the oscillation frequency is approximately 200 Hz. The droplet transition is observed when the dynamic equilibrium is broken. In this process, the Marangoni flow plays an important role in the droplet formation. Further, based on the transfer mode transformation mechanisms and a simple theory, a verified formula is proposed, according to which the energy required for maintaining the liquid bridge should be moderate. Therefore, the heat input conditions for maintaining the liquid bridge can be calculated quantitatively. This research is of considerable significance for understanding the physical processes in electron beam 3D printing and provides a promising avenue for process optimizations in industrial applications. (C) 2018 Elsevier Ltd. All rights reserved.