Flow characterization in a laser cut kerf

Abstract

Laser cutting is a thermal process that results in higher quality and precision than other thermal processes. In the cutting process, a focused laser beam melts and/or evaporates the material, and a gas jet, coaxial to the beam, blows the molten and/or vaporized material out of the kerf. Thus, not only the optical performance of the beam itself, but the characteristics of the gas jet, such as feed pressure, nozzle geometry, nozzle-workpiece separation, also control the cutting performance. In general cutting conditions, the gas jet becomes a supersonic underexpanded jet with shock waves in which the flow velocity and pressure along the nozzle axis rapidly change. The nozzle tip is placed /spl sim/1 mm upstream of the workpiece surface, forming an underexpanded impinging jet, or more complicated flow field. In the cutting processes, the gas jet impinges on the workpiece surface and penetrates into the narrow cutting kerf with its width of less than 1 mm, forming more complex flow structures. In the cutting kerf, the shearing and compressive stresses of the jet act on the irradiated- and heated-layer of the cutting front, and carry away the molten and/or vaporized material. These stresses are strongly dependent on the flow patterns of the jet in the kerf. It is very important to characterize the flow fields in the cutting kerf for the evaluation of the cutting performance, i.e. cutting speed and quality of cutting, of the gas jet, or the nozzle. Flow visualization and pressure distribution measurement were conducted to characterize the flow in the laser cut kerf for various conditions of the gas jets.