Improvement of slope angle of kerf in thick plate cutting of mild steel by fiber laser

Fiber laser cutting of thick mild steel plate with oxygen assist gas results in a large slope angle of kerf, and reduction of the slope angle of kerf is required to enhance the applications of fiber laser cutting. Therefore, the cutting characteristics of mild steel were discussed, and the improvement method of slope angle of kerf was experimentally and numerically investigated in the fiber laser cutting of mild steel. In the case of a large slope angle of kerf, heat is largely generated at lower part of thick plate, and reduction of the heat generation is necessary to achieve a small slope angle in fiber laser cutting of thick metal plate. When the tips of inner and outer nozzles in double nozzle configuration are arranged at the same level in a perpendicular plane to assist gas flow direction, the thickness of heat affected zone (HAZ) becomes small. A small slope angle can be achieved by using the nozzle, which can create higher and straighter gas flow. In fiber laser cutting of thick mild steel plate with oxygen assist gas, slope angle of kerf decreases with increasing the cutting speed. In other words, there is a large delayed distance of cutting front, which is the distance in scanning direction between tip and tail of front wall. In addition, a small delayed distance of cutting front indicates that reflected laser beam from the cutting front is absorbed on the both side walls at the rear of cutting front, while a large delayed distance leads to the reduction of reflected laser beam on the side walls. Computational fluid dynamics (CFD) analysis of assist gas flow suggests that sufficiently high flow velocity can push the molten material downward even in the case of large delayed distance of cutting front. In addition, oxygen density at lower part of plate decreases in the case of a large delayed distance, which can suppress the oxygen-combustion properly. Both phenomena can be achieved by good straightness of assist gas flow. Therefore, it is clarified that a small slope angle can be achieved by the combination of large delayed distance and assist gas supplying with good straightness of assist gas flow at lower part of thick plate.