Unstable thermocapillary convection in metal liquid bridge is a typical phenomenon during the laser metal-wire additive manufacturing process in microgravity environment. The evolution and dynamic mechanism of the liquid bridge will influence the manufacturing process and quality for the forthcoming on-orbit space metal additive manufacturing. Therefore, it is very important to investigate the evolution and instability of thermocapillary convection in liquid bridges in microgravity. In present investigation, a numerical model is developed to reveal the characteristics of thermocapillary convection. The effects of aspect ratio and gravity on the critical Reynolds number for convection instability of thermocapillary convection in metal (Ti6Al4V) liquid bridge are investigated numerically. The results indicate that the critical Reynolds number for convection instability decreases with the increase of aspect ratio number at first, and then increases both in the gravity or microgravity environment. The numerical results also reveal that the critical Reynolds number for convection instability under gravity environment with natural convection in metal liquid bridge is larger than microgravity environment. The research shows that the influence of microgravity leads to a distinctly different behaviour of thermocapillary convection in metal liquid bridge compared to the gravity environment. A more comprehensive study will be conducted to cover the parameter space more systematically to identify the factors which significantly influence the stability of the thermocapillary convection in metal liquid bridge under microgravity environment, which is important for the on-orbit space metal additive manufacturing.
Additive manufacturing in-space is considered to have the potential for achieving logistical support in future space exploration. In order to meet the future metal additive manufacturing in-space, our research team is developing an advanced metal-wire laser additive manufacturing technology. The laser system is composed by 8 laser beams in an annular array. In the manufacturing process, 8 laser beams focus on the substrate to create molten pool and the metal-wire is vertically fed into the molten pool. This technology has been evaluated on ground environment and the metallurgical microstructure of the fabricated metallic parts is studied. In the immediate future, we are planning to evaluate the technology in microgravity environment using aircraft parabolic flights and carry out the comparison experiments. The experiments are used to study the effects of microgravity on the molten pool behavior, metal parts geometry, microstructure and mechanical properties.
Additive manufacturing of metal parts in space is one of the potential means to realize on-orbit maintenance of aircraft. However, the basic phenomena such as the rapid melting and solidification behavior of metallic materials under the action of high-energy beams in space are unclear. It is necessary to observe those phenomena and reveal basic laws through space experiments. Therefore, an experimental platform for rapid melting and solidification of metal materials is developed. There are two parts included in this platform. A detailed design of the manufacturing system in space is described at first while the in lab experimental system on the ground is introduced also. In order to simulate the vacuum environment in space, a vacuum chamber is used to contain the core unit of the experimental system. Laser is used to melt a metal wire during the experiment while a positioning stage is adopted to shape the melted wire. The melting and solidification process is controlled automatically while it is monitored by a machine vision system at the same time.
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