选区激光熔化成型过程的球化现象研究

为获得高致密及高精度金属零件，弱化甚至消除选区激光熔化快速成型过程的球化现象十分必要。本文从理论上分析金属熔池球化演变机制以及弱化球化的方法，发现熔池的气液界面有使熔池发生球化的倾向；如对上一层基础有一定的重熔量，则熔池分为熔化粉末所形成的上部熔池以及熔化基础形成的下部熔池，下部熔池不会发生球化，且对上部熔池的球化趋势起阻碍作用；上部熔池的液固界面为熔液因重力下坠与基板界面形成的液固界面，视熔液与基板的润湿性的好或坏，对熔池球化具有促进或阻碍作用。因此，保证对上一层具有足够的重熔量可有效弱化球化现象。采用铜基合金粉末进行的单道熔池成型以及多层块状金属实体成型实验也验证了上述分析。

Balling Phenomenon in Selective Laser Melting Process

Selective Laser Melting (SLM) of metal powders is often accompanied by balling phenomenon which causes a rough and bead-shaped surface, obstructs a smooth layer deposition and decreases the density of the produced part. Thus, weakening balling phenomenon in SLM has become an important consideration in obtaining metal parts with near full-density and high accuracy. In this study, theoretical analysis has been done to explain the actual evolution mechanism and the weakening method of balling phenomenon. The analysis shows that: the liquid-gas contact surface always promotes the balling course. If the substrate is melted to a certain extent, the fresh molten pool includes two parts: the upper pool formed by the metal powder and the bottom pool formed by a part of the substrate. Balling doesn’t occur in the bottom pool, and the liquid metal in the bottom pool obstructs the balling course of the upper pool. The liquid-solid contact surface between the liquid metal, which is part of the upper molten pool and drops to the substrate surface due to gravity, and the substrate promotes the balling course if the liquid metal has poor wettability with the substrate or obstructs the balling course if has good wettability. Thus, during SLM processing, sufficient re-melting amount of the substrate can effectively weaken the balling phenomenon. The above analysis has been validated by some experiments carried out using Cu-based powder.