Fundamental study on reduction of dross in fiber laser cutting of steel by shifting nozzle axis

In the fiber laser cutting of steel with nitrogen assist gas, the effect of positional relationship between the centers of a nozzle and a laser beam on the cutting quality was investigated by laser cutting experiments and computational fluid dynamics analysis of the assist gas flow. The laser cutting experiment of 3.2 mm cold rolled steel was carried out by a 3 kW fiber laser. The spot diameter of the laser beam and the inner diameter of the nozzle were 140 μm and 2.0 mm, respectively. In laser cutting, by shifting the center of the laser beam toward the cutting direction inside the nozzle, the dross height becomes large. In the nozzle shifting setup, the cooling effect becomes high due to a large inflow of assist gas into the kerf, and the reduction of assist gas pressure results in the control of the cooling effect. The formation of appropriate kerf shape by changing the focusing position inside the workpiece leads to an appropriate flow field of assist gas in the kerf. As a result, similar dross height could be obtained by the nozzle shifting setup, compared with that by the coaxial setting of the nozzle and laser beam. The nozzle shifting setup can perform the equivalent dross height to the coaxial setting of the laser beam and nozzle even at lower pressure of the assist gas, and the reduction of assist gas consumption is possible at the same level of dross height in fiber laser cutting of steel.