GB2560738A

GB2560738A - Method of cutting a workpiece and assist gas composition

Info

Publication number: GB2560738A
Application number: GB1704573.3A
Authority: GB
Inventors: Fieret Jacob
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2017-03-23
Filing date: 2017-03-23
Publication date: 2018-09-26
Also published as: GB201704573D0

Abstract

The invention relates to a method for gas assisted laser cutting of a metal or metal alloy workpiece 20 utilising an assist gas 50, wherein the assist gas 50 comprises nitrous oxide (N2O). A laser beam 18 is directed to a cutting area 21 at the workpiece 20 and the assist gas 50 is supplied to the cutting area 21. The assist gas composition 50 comprising nitrous oxide is also claimed. The assist gas 50 may also comprise at least one inert gas such as nitrogen, argon or helium, and oxygen and/or at least one reactive gas such as organic fuel gases, acetylene and hydrogen. The assist gas 50 may be supplied from a storage container 30 in which the assist gas is stored in liquid form at room temperature. The storage device 30 may be refrigerated. The metal or metal alloy may comprise at least one of mild steel, iron and copper. The nitrous oxide is particularly suitable for use with fibre or disk lasers 15, since it does not interfere with the wavelength of the laser by partly absorbing the laser beam radiation, in the way that oxygen does at that wavelength.

Description

(54) Title of the Invention: Method of cutting a workpiece and assist gas composition Abstract Title: Method Of Cutting A Workpiece And Assist Gas Composition (57) The invention relates to a method for gas assisted laser cutting of a metal or metal alloy workpiece 20 utilising an assist gas 50, wherein the assist gas 50 comprises nitrous oxide (N₂O). A laser beam 18 is directed to a cutting area 21 at the workpiece 20 and the assist gas 50 is supplied to the cutting area 21. The assist gas composition 50 comprising nitrous oxide is also claimed. The assist gas 50 may also comprise at least one inert gas such as nitrogen, argon or helium, and oxygen and/or at least one reactive gas such as organic fuel gases, acetylene and hydrogen. The assist gas 50 may be supplied from a storage container 30 in which the assist gas is stored in liquid form at room temperature. The storage device 30 may be refrigerated. The metal or metal alloy may comprise at least one of mild steel, iron and copper. The nitrous oxide is particularly suitable for use with fibre or disk lasers 15, since it does not interfere with the wavelength of the laser by partly absorbing the laser beam radiation, in the way that oxygen does at that wavelength.

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Description

Method Of Cutting A Workpiece And Assist Gas Composition

Technical Field

The invention relates to a method for gas assisted laser cutting of metal or metal alloy workpieces as well as an assist gas composition useful therein.

Prior Art

Laser cutting uses a laser beam to heat up and melt material, and a jet (typically of high pressure) of assist gas (or assist gas composition) is used to remove the molten, combusted, or vaporised material from the cut. The material removal by the jet relies on the melt-shear removal of the rapidly moving assist gas.

A typical kind of laser used for laser cutting of, for example, metal is a carbon dioxide (CO₂) laser. In order to achieve a high cutting speed, oxygen can be used as assist gas for these kind of lasers since oxygen is a highly reactive but not hazardous gas. A typical wavelength of laser light produced by carbon dioxide lasers is 10.6 pm. A typical power level of such a laser is up to 15 kW and sometimes even higher.

Industrial oxygen can be supplied at a purity of typically 99.5%, whereby the remaining impurity consists mostly of the inert gases argon and nitrogen. The reason for using this purity of oxygen, rather than lower purity grades, or air, is that it allows for faster cutting.

Further kinds of lasers that are used in industrial applications for several years are fibre lasers or disk lasers. A fibre laser is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium.

They are related to doped fiber amplifiers, which provide light amplification without lasing. Fiber nonlinearities, such as stimulated Raman scattering or four-wave mixing can also provide gain and thus serve as gain media for a fiber laser.

A disk laser is a type of diode pumped solid-state laser characterized by a heat sink and laser output that are realized on opposite sides of a thin layer of active gain medium. The thickness of the disk is considerably smaller than the laser beam diameter.

Typical materials for which laser cutting with fibre lasers is used are in particular those metals which are suitable for laser cutting using oxygen as assist gas. These are metals for which the ignition temperature in oxygen is lower, or similar to, the melting temperature of the corresponding metal oxide, and whereby the metal oxide is liquid at the ignition temperature, and whereby the oxidation process of the metal is exothermal (releases heat). Under such conditions, laser cutting is particularly effective. Examples for materials are mild steel, iron, copper.

At the wavelength of the carbon dioxide laser, 10.6 pm, oxygen gas is completely or at least almost completely transparent, i.e. it does not absorb radiation at that wavelength. Contrary to the carbon dioxide laser, fibre and disk lasers operate at a much shorter wavelength, typically between 1.02 pm and 1.08 pm. At this wavelength, however, oxygen exhibits a major absorption band, centred at a wavelength of approximately 1.06 pm.

This means that oxygen as assist gas in laser cutting using fibre or disk lasers will partly absorb the laser beam radiation which can result in reduced cutting speed and quality.

The present invention aims to further improve efficiency of laser cutting, in particular when using fibre or disk lasers.

Disclosure of the Invention

This object is achieved by providing a method and an assist gas composition according to the independent claims.

The invention is based on a method for gas assisted laser cutting of a metal or metal alloy workpiece utilising an assist gas, wherein a laser beam is directed to a cutting area at the workpiece and the assist gas is supplied to the cutting area. Typically, the workpiece cut includes mild steel, iron or copper. Further, the workpiece used can have a thickness of at least 3 mm, preferably of at least 7 mm, particularly preferably of at least 10 mm.

Improving purity of oxygen or increasing the flow rate of oxygen as assist gas cannot remedy the problem that part of the laser light is absorbed by oxygen when the wavelength of laser light used is around an absorption band of oxygen. Increasing the power of the laser would not improve efficiency.

By using an assist gas comprising nitrous oxide, however, there does not occur any absorption of the laser light. Nitrous oxide (N₂O) releases oxygen on contact with the hot metal inside the laser cut. Under these circomstances, the oxygen immediately binds with the metal, forming the metal oxide, and there is therefore no, or a vanishingly small amount, of gaseous oxygen that could absorb laser light.

Nitrous oxide is stable below temperatures of 150° C, and is known not to produce detonation waves in gaseous (or liquid) form. Thus, this gas is safe to use as laser cutting assist gas. Although nitrous oxide has particular advantages over oxygen as assist gas when using fibre or disk lasers or a laser light with wavelength between 1.0 pm and 1.1pm, respectively, it can also be used with other types of lasers.

Preferably, the assist gas is supplied from a storage device in which the assist gas is stored in liquid form at room temperature or the assist gas is supplied from a refrigerated storage device. This means that the assist gas can be stored very easily, for example, in storage devices like tanks or cylinders

Further, as an added positive effect, molecular nitrogen is also released. For every molecule of nitrous oxide, one atom of oxygen and one molecule of nitrogen are created. As the released oxygen is in atomic form rather than molecular form, its reaction with metal is fast and more energetic than with molecular oxygen, where some energy is used to break the oxygen-oxygen bond in the molecule.

In practice, this means that at the cutting area, there is kind of an additional gas flow boost from the new nitrogen molecules created in the process. This is unique for laser cutting with nitrous oxide and does not occur with any other laser cutting assist or process gas. Therefore, laser cutting with nitous oxide can be expected to have several advantages over cutting with oxygen or an inert gas. This means that when using nitrous oxide, a deeper cut is possible than when using pure oxygen.

Preferably, the assist gas further comprises at least one inert gas, which, in particular, is chosen from nitrogen, argon and helium. Thus, a preferred concentration of nitrous oxide can be provided. Preferred concentations are, for example, between 0.001% and 95%, preferably between 10% and 90%. Advantageously, the assist gas can further comprise oxygen, which also allows effective combustion.

It is of further advantage, when the assist gas further comprises at least one reactive gas, which, in particular, is chosen from organic fuel gases, preferably hydrogen and acetylene. This allows the oxygen released by the nitrous oxide to react partly with the metal and partly with the other gas, which also produce heat and help to improve efficiency of the cutting process.

A further object of the invention is an assist gas composition for gas assisted laser cutting of a metal or metal alloy workpiece, which comprises nitrous oxide. Preferably, the concentration of nitrous oxide in the assist gas composition is between 0.001% and 95%, preferably between 10% and 90% (by weight).

In respect of further embodiments and advantages of the assist gas composition it is referred to the statements above in order to avoid repetition.

The invention will now be further described with reference to the accompanying drawings, which show a preferred embodiment.

Brief description of the drawings

Fig. 1 schematically shows a device, with which the method of the present invention can advantageously be implemented.

Detailed description of the drawings

In Fig. 1, a laser cutting device for cutting a metal workpiece 20, for example a mild steel workpiece, of thickness 26 is schematically shown and generally designated 100. The cutting of workpiece 20 is achieved by means of a cutting head 10. Cutting head 10 is provided with a suitable cutting laser 15, which emits a laser beam 18. Preferably, laser 15 is a fibre laser or a disk laser with a wavelength of laser light between 1.0 pm and 1.1pm.

Cutting head 10 comprises a housing 11 with a housing cavity 12. Via a gas inlet 13, an assist gas (or assist gas composition) 50 can be brought into cavity 12. Assist gas 50 is provided from a storage device, for example, a cylinder 30 and comprises nitrous oxide 51 and, preferably, also an inert gas 52 like, for example, nitrogen. Also, reactive gases can be present in assist gas 50.

A control computer 40 can be used to control cutting head 10, for example, a user can enter desired parameters, for example, a speed for relative displacement between cutting head 10 and workpiece 20.

The lower part of housing 11 is provided as a compound nozzle comprising a duct 14. Duct 14 is connected to cavity 12 and inlet 13, and serves to bring assist gas 50 onto the surface of workpiece 20 in the vicinity of cutting area 21, as will be described below.

A typical reaction between assist gas 50 and laser beam 18, which is focused onto cutting area 21 of workpiece 20 by means of a lens, will now be described. By means of laser beam 18, cutting area 21 of workpiece 20 is heated, which leads to a melting of the workpiece material at cutting area 21.

Furthermore, due to the heat generated by laser beam 18, nitrous oxide in assist gas 50 releases oxygen when in contact with the hot metal, in particular inside the cut. Oxygen then begins to combust the material leading to an additional heat generation in cutting area 21. At the same time, assist gas 50 leads to a removal of the molten material from cutting area 21 of workpiece 20, which is symbolized by arrow 25.

The process of cutting workpiece 20 along a cutting edge 22 is shown purely schematically. A kerf caused by laser beam 18 and the reactions of assist gas 50 due to the heat generated by the laser beam is designated 23. Front 27 of kerf 23 will typically be somewhat slanted under the influence of laser beam 18 and assist gas 50.

Claims

1. Method for gas assisted laser cutting of a metal or metal alloy workpiece (20) utilising an assist gas (50), wherein a laser beam (18) is directed to a cutting area (21) at the workpiece (20) and the assist gas (50) is supplied to the cutting area (21), characterised in that the assist gas (50) comprises nitrous oxide (51).

2. Method according to claim 1, wherein the laser beam (18) is generated by a fibre laser (15) or a disk laser.

3. Method according to claim 1 or 2, wherein laser light of a wavelength between 1.0 pm and 1.1 pm is used.

4. Method according to anyone of the preceding claims, wherein the assist gas (50) is supplied from a storage device (30) in which the assist gas (50) is stored in liquid form at room temperature or wherein the assist gas (50) is supplied from a refrigerated storage device.

5. Method according to anyone of the preceding claims, wherein the assist gas (50) further comprises at least one inert gas (52).

6. Method according to claim 5, wherein the at least one inert gas (52) is chosen from nitrogen, argon and helium.

7. Method according to anyone of the preceding claims, wherein a concentration of nitrous oxide (51) in the assist gas (50) is between 0.001% and 95%, preferably between 10% and 90%.

8. Method according to anyone of the preceding claims, wherein the assist gas (50) further comprises oxygen.

9. Method according to anyone of the preceding claims, wherein the assist gas (50) further comprises at least one reactive gas.

10. Method according to claim 9, wherein the at least one reactive gas is chosen from organic fuel gases, preferably hydrogen and acetylene.

11. Method according to anyone of the preceding claims, wherein the metal or metal

5 alloy used comprises at least one of mild steel, iron and copper.

12. Assist gas composition (50) for use in gas assisted laser cutting of a metal or metal alloy workpiece (20), characterised in that the assist gas composition (50) comprises nitrous oxide

10 (51).

13. Assist gas composition (50) according to claim 12, wherein the assist gas composition (50) further comprises at least one inert gas (52), which preferably is chosen from nitrogen, argon and helium.

14. Assist gas composition (50) according to claim 12 or 13, wherein a concentration of nitrous oxide (51) in the assist gas composition (50) is between 0.001% and 95%, preferably between 10% and 90%.

20

15. Assist gas composition (50) according to anyone of claims 12 to 14, wherein the assist gas (50) further comprises oxygen and/or at least one reactive gas, wherein the at least one reactive gas preferably is chosen from organic fuel gases, preferably hydrogen and acetylene.

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