Beam-Material Interactions and Dynamic Processing Characteristics in Laser Micro-thermal Materials Processing

利用各种光学测量方法和脉冲斩波器方法，研究了脉冲YAG激光材料加工过程中的光束-材料相互作用和动态加工现象。1）证明了脉冲激光作用于靶时产生的明亮火焰，即激光诱导羽流或等离子体，不是等离子体，而是低温、高密度的原子蒸气射流。羽流的光发射只发生在激光脉冲持续时间的早期，并且羽流在大部分时间内处于未激发态。当靶面辐照功率密度适中时，羽流是由双层结构组成的层流射流，即，当靶面辐照功率密度较大时，羽流的能量密度随靶面辐照功率密度的增大而增大，当靶面辐照功率密度较大时，羽流的能量密度随靶面辐照功率密度的增大而减小。蒸发原子的内核和压缩环境气体的外鞘层。当功率密度增大时，尾焰变成一股高度扰动的亚音速射流，并从靶中喷出一条细小的液体线，以飞溅的形式破碎成细小的颗粒。3）用高速摄影和脉冲斩波器方法研究了靶熔合区的时间变化。层流羽流形成时，熔池表面比较平坦，熔深主要由热传导发展。另一方面，当产生湍流羽流时，羽流的反冲压力使熔池深度降低，激光功率直接作用于暴露的熔合线边界，导致侵彻发展非常迅速。

Beam-material interactions and dynamic processing phenomena during a pulsed YAG laser materials processing have been clarified by various optical measurements and pulse chopper method. Main conclusions obtained in this study are summarized as follows:1) A bright flame, so called the laser induced plume or plasma, which generates at the pulse laser irradiation on a target is proved to be not a plasma but a low temperature and high density vapor jet of tarfet atoms. The light emission from plume takes place only at the early stage of laser pulse duration and the plume is in the unexcited state for most of the gation. Scattering loss of incident beam by ultra-fine particles becomes dominant after the decay of light emission.2) When the irradiation power density at the target surface is moderate, the plume is a laminar jet which is consisted of double layers, i.e., inner core of evaporated atoms and outer sheath layer of compressed ambient gas. while, When the power density becomes higher, the plume turns into a highly terbulennt subsonic jet and a fine thread of liguid is emerged from the target which beraks into fine particles as spattering.3) Temporal change of fusion zone of target has been clatified by high speed photographic study as well as pulse chopper method. At the time of laminar plume generation, the molten pool surface is fairly flat and the penetration develops mainly by the heeat conduction. On the other hand, when a terbulent plume is generated, the molten pool is deeply depressed by the recoil pressure of plume and the laser power is directly given to the exposed fudion boundary which leads very quick developmint of penetration.