Liquid motion in fusion welding plays the important role in the heat and mass transfer as well as stability of bead formation. In an arc weld pool, many works have been intensively conducted experimentally and theoretically. In a keyhole laser welding, however, there is few comprehensive study on the metal flow and its related phenomena in weld pool. In the present work, liquid motion in the laser weld pool was directly observed by the high-speed optical and X-ray transmission methods using tracing particles put in the pool. Slag motion on the pool surface revealed that there was a quick stream flowing around the side of keyhole and it changed the direction to forward at the rear part of weld pool. Namely, there was a big circulation of flow in the rear part of pool surface. Liquid flow inside the pool was observed by tracing the X-ray transmission images of Tungsten particles that were initially sandwiched between two thin sheets. The flow pattern inside the pool was very complex and there observed a strong eddy just behind the keyhole. The flow speed inside the pool was 0.3 to 0.5 m/s and this fast stream hindered the bubbles of metallic vapor and shielding gas to float up by buoyancy force and resulted in the formation of characteristic porosity.
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