Process for the laser beam cutting of strip or plate workpieces,

Details Process for the laser beam cutting of strip or plate workpieces, Process for the laser beam cutting of strip or plate workpieces,

1995-04-03

1996-11-26

Evans, Geoffrey S.

Electric heating

Metal heating

By arc

21912184, B23K 2612, B23K 2614

Patent

active

055782280

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a process for the laser beam cutting of strip or plate workpieces, more particularly magnetic steel sheets having a thickness of <1 mm, more particularly <0.5 mm, wherein a laser beam emitted by a laser beam source melts the workpiece with the formation of a vapour capillary at the cutting point and the melt is driven off by a cutting gas consisting of an inert gas/hydrogen mixture.
In conventional laser beam cutting use is made, for example, of a 1.5 kW CO.sub.2 laser and a standard focusing system, leading to intensities of approximately 10.sup.6 W/cm.sup.2 in the focusing zone. In such cutting a Fresnel absorption takes place at the cutting front of the metal with which any vaporization of the material can be ignored.
The laser beam cutting of metallic workpieces, more particularly the cutting of stainless steel sheet with a thickness of less than 1 mm is known from DE 36 19 513 A1. The active cutting was in the inert cutting gas is oxygen, representing a proportion of 30 to 90% by volume of the cutting gas. The oxygen is used for chemical exothermic reaction with the steel to generate additional thermal energy, so as to accelerate the cutting process. The oxygen also cooperates with the cutting gas to drive the melt produced in the cutting zone out of the cutting seam of the sheet. However, it has been found that the oxidation products cannot be completely driven out of the cutting seam, so that cutting edges substantially free from oxidation products cannot be obtained. Moreover, the chemical exothermic reaction produces a comparatively large heat affected zone laterally of the cutting zone, and this leads to damage to the workpiece, more particularly a metal sheet and any coating disposed thereon.
The parting of magnetic steel sheets has long been regarded as an advantageous field for the application of laser cutting. In one process of this kind (DE 27 43 544 A1) a CO.sub.2 laser having a power of several hundred watts is used for the cutting of magnetic steel sheets having a thickness of less than 1 mm. In that laser beam cutting process the workpiece is acted upon at the place of parking by an oxygen-containing gas emitted from a nozzle, the pressure upstream of the nozzle being over 5 bar. In such cutting, cutting speeds of just 10 m/min can be reached, but such speeds are of no economic advantage in the case of magnetic steel sheets. Moreover, due to the resulting heating of those zones of a more particularly grain-oriented magnetic steel sheet which are adjacent to the cutting seam, such low speeds have an undesirable effect on the sheet's magnetic properties. Specialists in the field therefore consider that it is unsuitable to use high power lasers, more particularly a CO.sub.2 laser, with which the workpiece is to be cut preferably using oxygen and/or nitrogen as the cutting gas, for the parting of magnetic steel sheets, more particularly grain-oriented magnetic steel sheets ("Stahl und Eisen" 110 (1990), No. 12, pages 147-153).
However, various arguments and experiments are to be found elsewhere which are aimed at higher speeds in the laser beam cutting of workpieces. For example, WO 88/01553 discloses the offsetting of the axis of the laser beam, which is enclosed by a nozzle for supplying gas to the cutting point, in relation to the axis of the pressure center of the gas nozzle, namely in the direction of the uncut sheet. This is based on the idea that the material must first be melted, so that it can then more satisfactorily be driven out of the cutting seam at a subsequent place. However, that kind of laser beam cutting cannot be successfully used with thin sheets, if the intention is to perform cutting at high speeds.
In a recently disclosed process for laser beam cutting of the kind specified ("DVS-Berichte" 135, 1991, pages 12 to 15) cutting speeds of approximately 100 m/min can be reached with a sheet thickness of 0.25 mm, using an iron alloy as the material, when a given kind of laser beam, preferably a CO.sub.2 laser is used, with optimally harmonized

REFERENCES:
Plasma Effects in Laser Beam Cutting-D. Petring, K. U. Priessig, H. Zefferer and E. Beyer, Aachen, DVS-Berichte, 1991, vol. 135, pp. 12-15.

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