Laser welding in vacuum with a high-power laser is performed for a stainless steel plate which forms a keyhole inside the molten pool, resulting in a deep penetration without spatters. The spatter is one of the defect factors for laser welding, such as thinning of weld bead, and pore generation. In order to elucidate the mechanism of spatter free process, in situ observations were conducted to investigate dynamics of molten pool and keyhole in vacuum laser welding with a highspeed video camera and X ray transmission system. A stainless steel type 304 (SS304) is widely used in several industries such as automotive industry, chemical plants, petrochemical industry, etc. due to its excellent properties such as high corrosion resistance, hardness, low-temperature toughness, and thermal stability. The stainless steel type 304 (SS304) was set in a vacuum chamber and then a disk laser with an output power of 6 kW was irradiated and scanned on it to form a weld bead under aimed pressure. At the same time, the behavior of molten pool and keyhole were captured with real time observation system and an X ray transmission system, respectively. From the results, it was found that this fluctuation of molten pool affected the generation of spatter.
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