Method for flame cutting using a laser beam

Abstract

The invention relates to a method for flame cutting a workpiece (2) by means of a laser beam (3), the laser beam having a power of more than 10 kW, the workpiece having a thickness (D) of at least 10 mm and being in the form of a plate, and the method using oxygen as cutting gas. According to the invention, the focal position (F) of the laser beam (3) in the beam direction (6) is at a depth in the workpiece (2) greater than half (D / 2) of the thickness (D) of the workpiece (2), and the laser beam (3) emerges together with the cutting gas from a nozzle opening (5) of a cutting gas nozzle (1), wherein the distance (A) between the nozzle end face (8) on the workpiece side and the workpiece surface (9) is at least 2 mm, preferably at least 3 mm, particularly preferably at least 5 mm.

Description

Technical Field

[0001] The present invention relates to a method for flame cutting a workpiece with a laser beam having a power of more than 10 kW, the workpiece having a thickness of at least 10 mm and being in the form of a plate, and using oxygen as the cutting gas. In this flame cutting method, the laser beam is typically moved relative to the workpiece in a (usually variable) cutting direction, with the cutting gap being formed in the workpiece opposite to the cutting direction. Background Art

[0002] For cutting relatively thick workpieces, a relatively large focal diameter is generally desirable for the machining laser beam. The cutting gap should be wide enough to allow for the removal of liquefied workpiece material and / or slag generated during cutting. In contrast, a relatively small focal diameter is desirable when machining relatively thin workpieces, particularly for high-speed laser cutting.

[0003] WO 2011 124671 A1 and WO 2014 060091 A1 disclose switching one or more solid-state laser beams between the different cores of a multi-core optical fiber to produce a laser beam with variable laser beam characteristics at the fiber output. This laser beam can be focused on the workpiece to be cut with a variable focal diameter by means of a downstream cutting head. For example, when cutting at very high laser powers exceeding 10 kW, the described system enables high-quality cutting of both thick and thin workpieces. However, within this power range, an increase in laser power is no longer likely to result in a corresponding increase in cutting speed, i.e., feed motion.

[0004] WO 2009 007708 A2 provides preferred process parameters for flame cutting of sheet metal using oxygen as the cutting gas. For example, the laser beam focal point should be positioned above the sheet metal surface, specifically at a distance of approximately 4 to 5 mm from the sheet metal surface. The distance between the sheet metal surface and the processing nozzle should be between approximately 1 and 2 mm. Summary of the Invention

[0005] The object of the present invention is to provide a method for flame cutting by means of a laser beam having a laser power of more than 10 kW, in which method an increase in the cutting speed can be achieved.

[0006] This object is achieved by a method of the type mentioned at the outset, in which the focal position of the laser beam in the beam direction is located or positioned at a depth in the workpiece that is greater than half the thickness of the workpiece, and in which the laser beam is emitted together with the cutting gas from the nozzle opening of the cutting gas nozzle, wherein the distance between the nozzle end face on the workpiece side and the workpiece surface is at least 2 mm, preferably at least 3 mm, particularly preferably at least 5 mm.

[0007] In other words, the focal point of the laser beam is at a distance greater than half the workpiece thickness from the workpiece surface at which the laser beam enters the workpiece. If the laser beam impinges on the workpiece from the top, as is typically the case, the focal point, i.e., the position of the laser beam waist, is located below the center of the workpiece. The focal point is not typically located below the workpiece; that is, the laser beam is focused at a focal point between half the workpiece thickness and the full workpiece thickness. The greater the workpiece thickness, the greater the distance between the focal point and the workpiece surface.

[0008] This setting of the focal position deviates significantly from the settings used hitherto, in which the focal position is arranged at the top side of the workpiece, slightly below it or above it (see WO 2009 007708 A2).

[0009] With the type of focusing according to the present invention, the focal point is located very deep in the workpiece. This extremely deep focal position causes the laser beam to defocus at the workpiece surface, resulting in a decrease in the power density at the workpiece surface. This is accompanied by a widening of the cutting gap. In this way, when increasing the laser power in the range of 10 kW to 20 kW, it is surprisingly possible to achieve a significant and continuous increase in cutting speed while maintaining good cut edge quality and process reliability. For example, when flame cutting thick workpieces using conventional cutting parameters, a 50% increase in laser power results in a feed rate increase of less than 20%. However, with the method according to the present invention, a surprising 50% increase in feed rate can also be achieved with a 50% increase in power.

[0010] The inventors have recognized that it is also advantageous for the implementation of this method to set a very large distance between the cutting gas nozzle, or more precisely, the nozzle end face, and the workpiece surface, because this facilitates the desired effect of increasing the feed rate with increasing laser beam power. Selecting a large distance between the cutting gas nozzle and the workpiece surface also contradicts the technical teaching of WO 2009 007708 A2, which states that the distance between the nozzle and the top side of the workpiece should be between 1 mm and 2 mm.

[0011] In another embodiment, the laser beam is generated in a laser beam generator that is connected to a cutting head via an optical fiber, to which a cutting gas nozzle is mounted. The optical fiber is designed as a single-core fiber or as a multi-core fiber. The optical fiber can be designed as described in WO 2011 124671 A1, that is, as a multi-clad fiber with an inner fiber core and at least one annular core. The multi-core fiber can also be designed as described in WO 2014060091 A1. When using the process parameters according to the present invention, the use of a multi-core fiber is possible, but no longer absolutely necessary; rather, the fiber can have only a single core, as is common with simple or conventional optical fibers.

[0012] In another variant, the single-core optical fiber has a core diameter between 50 μm and 150 μm. Core diameters in this range have proven to be advantageous for flame cutting. Typically, the laser beam emitted from the optical fiber is focused onto the workpiece by a focusing device arranged in the cutting head, for example in the form of focusing optics, such as a focusing lens.

[0013] In one variant, the laser beam has a Gaussian intensity distribution on the upper side of the workpiece. This Gaussian intensity distribution has proven to be advantageous for flame cutting with the aforementioned parameters. Typically, a Gaussian intensity distribution is present when the laser beam emerges from a single-core optical fiber, so that when using such an optical fiber, no additional optical elements are required to generate the Gaussian intensity distribution.

[0014] In another embodiment, the focal diameter of the laser beam at the focal point is between 150 μm and 300 μm, preferably 200 μm. This focal diameter has proven to be advantageous for flame cutting of thick plate-like workpieces, in particular sheet metal, when the focal point is in the lower half of the workpiece.

[0015] In another variant, the laser beam is generated using a solid-state laser or a diode laser as the laser beam generator. Solid-state lasers and diode lasers have proven advantageous for rapid cutting, particularly of thin workpieces, and offer better energy efficiency than CO2 lasers. The method according to the invention significantly expands the application areas of solid-state lasers or diode lasers to include flame cutting processes.

[0016] In another variant, the overpressure (container pressure) of the cutting gas (oxygen) before exiting the nozzle opening is between 0.4 bar and 1 bar. Due to the higher laser powers available, exceeding 10 kW, less oxygen is required to achieve a uniformly exothermic combustion process. Excessive oxygen levels can lead to uncontrolled burning of the cut seam.

[0017] Further advantages and advantageous configurations of the subject matter of the invention are apparent from the description, the claims, and the drawings. Likewise, the features mentioned above and those to be listed below can be used individually or in any desired combination of multiple features. The exemplary embodiments shown and described are not to be understood as an exhaustive list, but rather have an exemplary character for outlining the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In the accompanying drawings:

[0019] Figure 1 shows a longitudinal section through a cutting gas nozzle and a plate-like workpiece during flame cutting with the aid of a laser beam;

[0020] Figure 2 a graph showing the dependence of the laser beam focus position on the workpiece thickness;

[0021] Figure 3 A laser cutting machine for carrying out the method for flame cutting is shown. DETAILED DESCRIPTION

[0022] In the following description of the figures, the same reference numerals are used for identical or functionally identical components.

[0023] Figure 1 A cutting gas nozzle 1 is shown for laser cutting a sheet metal workpiece 2 (sheet metal) having a thickness D of at least 10 mm by means of a laser beam 3 and a cutting gas 24 (see FIG. Figure 3 ). Cutting gas 24 and laser beam 3 are both emitted from nozzle opening 5 of cutting gas nozzle 1. Laser beam 3 has a beam direction 6 extending along the negative Z-axis direction of the XYZ coordinate system. The laser cutting process is a flame cutting process using oxygen as cutting gas 24.

[0024] The cutting gas nozzle 1 is moved above the workpiece 2 in a cutting direction 7, which corresponds to the X direction of the XYZ coordinate system, in order to produce a cutting gap in the workpiece 2. In the example shown, the distance A from the nozzle end face 8 on the workpiece side to the workpiece surface 9 facing the cutting gas nozzle 1 is at least 2 mm, preferably at least 3 mm, and in particular at least 5 mm. The focal position F of the laser beam 3 in the beam direction 6 is located within the thickness D of the workpiece 2, more precisely in the lower half of the workpiece 2 facing away from the cutting gas nozzle 1. In other words, the focal position F of the laser beam 3 in the beam direction 6 is located at a depth in the workpiece 2 that is greater than half the thickness D of the workpiece 2, D / 2. In this case, the focal diameter d at the focal position F in the workpiece 2 is F Between 150 μm and 300 μm, preferably approximately 200 μm.

[0025] Figure 2The graph in FIG. 1 shows the relationship between the focal position (in millimeters) in the workpiece 2 (sheet metal) and the workpiece thickness (sheet metal thickness) (in millimeters). It can be seen that the thicker the workpiece 2, the deeper the focal position F is located in the workpiece 2. Thus, the greater the workpiece thickness D, the greater the distance between the focal position F and the workpiece surface 9.

[0026] Figure 3 A laser cutting machine 20 suitable for carrying out the flame cutting method described above is shown.

[0027] The laser cutting machine 20 has a solid-state laser or a diode laser as a laser beam generator 21. The laser cutting machine 20 also has a movable (laser) cutting head 22 and a workpiece holder 23, on which the workpiece 2 is arranged. A laser beam 3 is generated in the laser beam generator 21 and is guided from the laser beam generator 21 to the cutting head 22 by means of an optical fiber (not shown). In the example shown, the optical fiber is a single-core optical fiber, that is, the optical fiber has only one core, in which the laser beam 3 or the laser radiation of the laser beam generator 21 propagates. In the example shown, the single-core optical fiber has a core diameter between 50 μm and 150 μm. Alternatively, a multi-core optical fiber can also be used to guide the laser beam 3 from the laser beam generator 21 to the cutting head 22.

[0028] The laser beam 3 is directed toward the workpiece 2 by means of focusing optics arranged in the cutting head 22. The laser beam 3 emitted from the single-core optical fiber has a Gaussian intensity distribution and maintains this Gaussian intensity distribution when focused on the workpiece 2. In other words, the laser beam 3 also has a Gaussian intensity distribution on the workpiece surface 9.

[0029] Furthermore, the laser cutting machine 20 is supplied with a cutting gas 24, which here is shown as an example of oxygen or nitrogen. To carry out the flame cutting method described above, oxygen is supplied as cutting gas 24 to the cutting gas nozzle 1 of the cutting head 22, specifically at an overpressure of approximately 0.4 bar to 1.0 bar before the cutting gas 24 is emitted from the cutting gas nozzle 1.

[0030] The laser cutting machine 20 also includes a machine controller 25, which is programmed to move the cutting head 22, including the cutting gas nozzle 1, relative to the stationary workpiece 2 according to the cutting contour. The machine controller 25 also controls the power of the laser beam generator 21, which, in the flame cutting process described above, is greater than 10 kW and can be as high as 20 kW or more, if necessary. This allows, for example, a cutting speed (feed) of 3.1 m / min for a workpiece thickness of 15 mm and a cutting speed of 1.75 m / min for a workpiece thickness of 25 mm, with the cutting speed increasing as the laser power increases.

[0031] Reference Signs List

[0032] 1 Cutting gas nozzle

[0033] 2 Workpiece

[0034] 3 Laser beam

[0035] 5 Nozzle opening

[0036] 6 Laser beam direction

[0037] 7 Cutting direction

[0038] 8 Nozzle end face

[0039] 9 Workpiece surface

[0040] 20 laser cutting machines

[0041] 21 Laser beam generator

[0042] 22 cutting head

[0043] 23 Workpiece receiving unit

[0044] 24 Cutting gas

[0045] 25 Machine Controller

[0046] F Focus position

[0047] D Workpiece thickness

[0048] A Distance

[0049] d F Laser beam diameter

Claims

1. A method for flame cutting a workpiece (2) by means of a laser beam (3), said laser beam (3) having a power of more than 10 kW, said workpiece (2) having a thickness (D) of at least 10 mm, said method using oxygen as cutting gas (24), It is characterized in that The focal position (F) of the laser beam (3) along the beam direction (6) is at a depth in the workpiece (2) that is greater than half (D / 2) of the thickness (D) of the workpiece (2), and The laser beam (3) is emitted together with the cutting gas (24) from a nozzle opening (5) of a cutting gas nozzle (1), wherein the distance (A) between the nozzle end face (8) on the workpiece side and the workpiece surface (9) is at least 2 mm.

2. The method according to claim 1, characterized in that The laser beam (3) is generated in a laser beam generator (21), which is connected to a cutting head (22) via an optical fiber, and the cutting gas nozzle (1) is mounted on the cutting head, wherein the optical fiber is configured as a single-core optical fiber or a multi-core optical fiber.

3. The method according to claim 2, characterized in that The single-core optical fiber has a core diameter between 50 μm and 150 μm.

4. The method according to any one of claims 1 to 3, characterized in that The laser beam (3) has a Gaussian intensity distribution on the workpiece surface (9).

5. The method according to any one of claims 1 to 3, characterized in that The focal diameter (d) of the laser beam (3) at the focal position (F) F ) is between 150μm and 300μm.

6. The method according to any one of claims 1 to 3, characterized in that The laser beam (3) is generated by means of a solid-state laser or by means of a diode laser as a laser beam generator (21).

7. The method according to any one of claims 1 to 3, characterized in that Before exiting the cutting gas nozzle (1), the overpressure of the cutting gas (24) is between 0.4 bar and 1 bar.

8. The method according to any one of claims 1 to 3, characterized in that The workpiece (2) is plate-shaped.

9. The method according to any one of claims 1 to 3, characterized in that The distance (A) is at least 3 mm.

10. The method according to any one of claims 1 to 3, characterized in that The distance (A) is at least 5 mm.

11. The method according to any one of claims 1 to 3, characterized in that The focal diameter (d) of the laser beam (3) at the focal position (F) F ) is 200μm.

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