Characterization of a line-shaped molten iron pool produced by a line-focused single-mode CW laser with a prism technique

We perform experimental characterization of a line-shaped molten pool produced by a single-mode continuous-wave fiber laser at a 1070 nm wavelength coupled with line-focusing optics, wherein heat conduction-type melting occurs with gentle hydrodynamics. This causes sputterless melting. To produce the desired shape of a molten pool along the line-focus direction using an intensity profile modification technique for a single-mode laser, we used a prism coupled with a pair of cylindrical and spherical lenses. A nickel-coated cold-rolled steel sheet was used as the target. High-speed cameras synchronized with laser irradiation were used to record the molten pool dynamics on the laser-irradiated side and those on the rear side simultaneously. Laser-intensity-dependent dynamic processes of line-shaped molten pools were observed, wherein two molten pools counterpropagate to form a single one. When the laser intensity was ∼2 × 104 W/cm2, the melting rise times and molten-pool coalescence times were delayed compared with those at an intensity of ∼6 × 104 W/cm2 owing to the lower heating rate. Although the evolution of a molten pool initially depends on the laser irradiation intensity profile, the profile does not directly induce the shape of the molten pool, but the shape is significantly modified by thermal conduction and hydrodynamic motion. The prism technique results were experimentally characterized and demonstrated as a promising shaping control technique for producing a line-shaped molten pool without scanning irradiation.