In order to understand laser keyhole welding phenomena, pulsed or continuous wave laser was irradiated on a molten metal of Sn or Zn, and spattering and keyhole evolution were observed by high-seed video camera and X-ray transmission method, respectively. It was confirmed from the observation of the surface that a keyhole was initiated to form by far earlier in the molten metal than in the solid metal. According to the X-ray transmission real-time observation result in Zn liquid metal, bubbles were predominantly generated from the tip of a keyhole, which is the same formation mechanism as we revealed in general laser welding. Furthermore, simplified numerical calculation demonstrated that surface tension should affect the formation of such bubbles.
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