Dynamic coupling between molten pool and metallic vapor ejection for fiber laser welding under subatmospheric pressure

The interaction between molten pool and metallic vapor ejection is of vital importance for weld bead formation. This is especially the case for the penetration depth in fiber laser welding under subatmospheric pressure. To present the dynamic coupling mechanism and the effect of subatmospheric pressure on the molten pool, a series of welding experiments were performed at the following pressures: 101, 40, 20, 10 and 3 kPa. The coupling mechanism between the melt flow and the metallic vapor was analyzed by observing their behavior using a high-speed camera. As the ambient pressure decreased from 101 kPa to 3 kPa, the molten pool width and the keyhole diameter decreased. In addition, the weld bead surface became narrow and smooth. However, in a normal atmosphere, the surface was wide and rough with V-shaped ripples. The fluctuation of the molten pool was in accordance with the metallic vapor ejection behavior. An uphill would form on the molten pool surface whenever the metallic vapor was ejected to the front side of the keyhole. In contrast, a downhill emerged when the metallic vapor was ejected at the rear side of the keyhole. Under subatmospheric pressures, especially below 10 kPa, the smooth weld bead surface was obtained because of the stable ejection of metallic vapor. This also resulted in smaller fluctuations in uphills and downhills within a narrow range on the molten pool surface.