Effects of metal-vapor jet force on the physical behavior of melting wire transfer in electron beam additive manufacturing

•Effects of metal-vapor jet force on transfer were related to transition height.•Physical behavior of melting wire transfer was investigated.•Metal transfer was unstable with poor weld appearance.•Weld appearance of big-droplet transfer was better than that of metal transfer.•Molten-metal-bridge transfer was proved to be the most stable transfer mode.

The effects of metal-vapor jet force on the physical behavior of melting wire transfer in electron beam additive manufacturing were analyzed by both means of theoretical and experimental studies. The maximum size of the droplet in metal transfer was calculated by the static force balance model theoretically. Meanwhile, the transfer behavior was observed by using a charge-coupled device visual system. The calculated maximum size of the droplet matched well with the experimental data, indicating that the model could effectively predict the critical size of the droplet. Our results revealed three transfer modes induced by different transition heights: metal transfer, big-droplet transfer and molten-metal-bridge transfer. There were two stages in metal transfer at the high transition height of 10 mm. In the first stage, the droplet with a radius of 1.5 mm flew horizontally and dripped outside the molten pool because of the horizontal component of the metal-vapor jet force. In contrast, in the second stage, the droplet with a radius of 3.93 mm flew vertically and dripped inside the molten pool with the reason of the vertical component of the metal-vapor jet force. When the transition height was reduced to 4 mm, the big-droplet transfer occurred, which was similar to metal transfer. As the transition height decreased further, the molten-metal-bridge transfer mode appeared. This was the most stable transfer mode. In addition, the mechanism of each transfer mode was also proposed. The metal-vapor jet force was proved to play an important role in droplet growth and stability. Finally, it is essential to decrease the transition height to avoid the metal-vapor jet force from influencing the stability of process.