Recently, a fiber laser has superseded CO₂ laser as a main laser source because of its excellent electro-optical conversion efficiency. However, there is a large dross remained on the back surface of workpiece in fiber laser cutting, compared with CO₂ laser. Thus, the reduction of dross is strongly required to improve the cutting quality in fiber laser cutting. In general, the laser cutting is carried out by arranging the centers of the nozzle and the laser beam coaxially. On the other hand, the efficient use of assist gas injected from the nozzle is expected by shifting the centers of laser beam and nozzle. If the amount of dross is reduced with low assist gas consumption, the reduction of running cost can contribute to spread the applications of fiber laser cutting.

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