激光快速成形过程中熔池形态的演化

Evolution of Molten Pool Shape in the Process of Laser Rapid Forming

采用高速摄影技术对激光快速成形过程中液态熔池的形成及其演化过程进行了实时观察。结果发现,以一定速度向前运动的激光束辐照基材时,基材表面开始熔化并形成液态熔池,经过一个较短的时间间隔(约1．0 s)后熔池深度增大至一定值,熔池长度则围绕一恒定值波动。以恒定送粉率向熔池中连续送进金属粉末时,熔池的长度和宽度逐渐减小,熔池寿命缩短。同时熔池后沿不断抬高即熔覆层厚度不断增加,最大熔深处熔池自由表面法向和激光束轴线之间的夹角由几度逐渐增大到20°～30°左右。熔池自由表面发生周期性的变化,实验观察到熔池后沿有周期性的“岛状凸起”出现和消失现象。数值计算结果证实这主要是熔池中熔体在表面张力梯度下引起的强制对流作用的结果。

The high-speed photography has been employed to realize in situ observation on the formation and evolution of the molten pool in the process of laser rapid forming. The results show that the substrate begins to melt and produce a liquid molten pool with the laser beam scanning on it. After a rather short time （about 1.0 s） the molten pool depth increases to a certain value while the molten pool length fluctuates surrounding an invariableness figure. The depth and width of the molten pool decrease gradually with a constant flow rate powder being fed into the molten pool. And the lifespan of the molten pool turns to be short. Simultaneously the tail of the molten pool runs up that means the thickness of the cladding layer increases continuously with the powder ejecting into the molten pool. The angle between the normal of the free surface of the molten pool and the laser beam axis increases from a few degrees to 20-30 degree. There is an ＂island heave＂ emerging and vanishing seasonally at the tail of the molten pool free surface by in situ observation. This is the result of forced convection on the fused liquid of the molten pool induced by surface tension gradient, which has been proved by numerical simulation.