Recently, fiber laser cutting with oxygen assist gas has been widely used for thick plate cutting of mild steel. However, the slope angle of kerf becomes large in fiber laser cutting, although CO2 laser cutting can obtain a small slope angle of kerf. The mechanism to generate a large slope angle of kerf has not been clarified yet, and the reduction of slope angle is required in thick plate cutting of mild steel by fiber laser with oxygen assist gas.
Therefore, the differences in cutting characteristics between CO2 and fiber laser were experimentally investigated using thick plate of mild steel, and their cutting mechanisms were discussed. The re-solidified layer by the fiber laser is thicker than that by CO2 laser, and its thickness becomes large at lower part of plate. The fiber laser shows thicker heat affected zone (HAZ) than CO2 laser, and increasing rate of HAZ thickness by the fiber laser is higher than that by CO2 laser. These results indicate that heat generation is relatively large at lower part of thick plate in the case of large slope angle of kerf by the fiber laser, and the reduction of heat generation at lower part is required to decrease the slope angle of kerf in fiber laser cutting of thick plate.
Assist gas flow was numerically analyzed to achieve a small slope angle in the fiber laser cutting using coaxial nozzles of different shapes, which have inner and outer nozzle as double configuration. A small slope angle of kerf can be achieved when the tips of inner and outer nozzles in double nozzle configuration are positioned at the same level at perpendicular plane to assist gas flow direction. Hence, higher and straighter gas flow can be performed, and heat generation becomes lower, which results in the reduction of HAZ thickness.