Aerodynamic Interactions During Laser Cutting

Abstract

Most laser cutting systems utilise a gas jet to remove molten or vaporised material from the kerf.The speed, economy and quality of the cut can be strongly dependent on the aerodynamic conditions created by the nozzle, workpiece proximity and kerf shape. Adverse conditions can be established that may lead to an unwelcome lack of reproducibility of cut quality.Relatively low gas nozzle pressures can result in supersonic flow in the jet with its associated shock fronts.When the nozzle is placed at conventional distances (1 -2mm) above the workpiece, the force exerted by the gas on the workpiece and the cut products (the cutting pressure) can be significantly less than the nozzle pressure. Higher cutting pressures can be achieved by increasing the height of the nozzle above the workpiece, to a more damage resistant zone, provided that the shock structure of the jet is taken into account.Conventional conical nozzles with circular exits can be operated with conditions that will result in cutting pressures up to 3 Bar (g) in the more distant zone. At higher pressures in circular tipped nozzles the cutting pressure in this zone decays to inadequate levels.Investigations of a large number of non -circular nozzle tip shapes have resulted in the selection of a few specific shapes that can provide cutting pressures in excess of 6 Bar(g) at distances of 4 to 7mm from the nozzle tip.Since there is a strong correlation between cutting pressure and the speed and quality of laser cutting, the paper describes the aerodynamic requirements for achieving the above effects and reports the cutting results arising from the different nozzle designs and conditions. The results of the work of other investigators, who report anomalous laser cutting results, will be examined and reviewed in the light of the above work.

Gas investigation for laser drilling

“…As Fieret et al showed 7,8 that the surface pressure depends on the distance between the exit nozzle and the target surface. Striosopy images can evidence so.…”

“…Figure 6 presents the total pressure on target surface ͑and so, on the front of hole during drilling͒ as function of WD. As shown by Fieret,7,8 curves correspond to typical profiles of a jet without MSD. Pressure decreases for WD coming from 0 to 3.5 mm.…”

“…7,8 Consequently, a good protection of exit nozzle from degradation or/and a regular nozzle characterization are primordial. For example, for the present experiment, to protect optics and nozzle and to ensure a constant pressure on target, a working distance higher than 10 mm could be quite a good compromise.…”

“…Besides, these images show that for P noz above 2 bars, the Mach shock disk ͑MSD͒ is missing with this nozzle, only the Prandtl reflections appear. 6,7 They are generated by some imperfections of the lining nozzle. MSD lack shows that the nozzle is correctly shaped at these pressures.…”

A TWO-DOF Controlled Lens Drive Actuator for Off-Axis Laser Beam Cutting

“…However, the thicker the workpiece is, the lower the capability of the assist gas jet has in removing molten material from the cutting front. Insufficient gas flow through the cutting front decreases the cutting speed, and generates burr, dross and recast layers (3)- (5) . In order to improve the cutting speed and the capability to remove molten material, and, furthermore, to save gas consumption, real-time positioning control of the relative displacement between the lens and the assist gas jet nozzle in laser beam cutting machines (off-axis control) has been proposed, as shown in Fig.…”