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

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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.