A three-dimensional analysis of gas entrainment operating during the laser-cutting process

Abstract

Recent work has shown the great sensitivity of the reactive-gas-assisted laser-cutting process to impurities in the assisting gas. Process improvements have been obtained with high-purity oxygen jets and several companies have marketed a high-purity oxygen assisting gas. The role of impurities in limiting reaction energy in the laser-cutting process has aroused great interest in laser-processing laboratories, and is the subject of continuing research. In this work the authors have examined a traditional laser-cutting arrangement in order to assess the levels of impurity entrainment into the main oxygen gas jet. A three-dimensional theoretical analysis of the turbulent gas flow from a 1.5 mm diameter circular orifice through a model cut was examined in order to determine the magnitude of impurity entrainment as a function of kerf width and cut depth. Kerf widths in the range 0.4-1.4 mm and cut depths 0-20 mm were chosen as representative of possible cutting conditions. The onset of impurity entrainment at the model cut front occurred at a depth of 6.5 mm for kerf width 1.4 mm, and at a depth of 20 mm for kerf width 0.48 mm. Calculated entrainment levels are given for each combination of kerf width and cut depth within the chosen range. To examine the validity of the theoretical calculations, an experimental investigation of the benefits of using an annular jet, to protect against impurity entrainment, in conjunction with the standard cutting jet was made during CO2 laser cutting trials. Laser cuts were performed on 3, 5, 10, 16 and 20 mm thick 43A medium carbon steels with and without the anti-entrainment nozzle assembly. Cutting results support general theoretical results in that entrainment is not a significant problem for cut depths up to 10 mm thick and kerf widths less than 1 mm. A suitable anti-entrainment nozzle assembly will eliminate impurity entrainment beyond this range.