A 3-DIMENSIONAL ANALYSIS OF GAS ENTRAINMENT OPERATING DURING THE LASER-CUTTING PROCESS

Recent work has shown the great sensitivity of the reactive-gas-assist ed laser-cutting process to impurities in the assisting gas. Process i mprovements have been obtained with high-purity oxygen jets and severa l companies have marketed a high-purity oxygen assisting gas. The role of impurities in limiting reaction energy in the laser-cutting proces s has aroused great interest in laser-processing laboratories, and is the subject of continuing research. In this work the authors have exam ined a traditional laser-cutting arrangement in order to assess the le vels of impurity entrainment into the main oxygen gas jet. A three-dim ensional 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 dept hs 0-20 mm were chosen as representative of possible cutting condition s. The onset of impurity entrainment at the model cut front occurred a t 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 ex amine the validity of the theoretical calculations, an experimental in vestigation of the benefits of using an annular jet, to protect agains t impurity entrainment, in conjunction with the standard cutting jet w as 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. Provi sion of a suitable anti-entrainment nozzle assembly will eliminate imp urity entrainment beyond this range.