Patent Application
20230111969
The invention relates to a method for laser cutting a workpiece having a thickness of less than 6 mm. The invention furthermore relates to a laser cutting apparatus for laser cutting an, in particular three-dimensionally shaped, metal-sheet-type workpiece along a three-dimensional cutting line.
As focus diameters become increasingly smaller, the feed rate (the cutting speed) during laser cutting can be increased with the same laser power. However, this is limited by the fact that, if the focus is too small, the cutting quality becomes unacceptable. In particular, burr formation takes place. This burr formation is caused by the fact that less and less cutting gas penetrates into the cutting kerf as the cutting kerf gets smaller and smaller, and so it is not ensured that the molten metal is ejected.
For this reason, endeavors have been made in recent years primarily to influence the beam properties when cutting increasingly thicker workpieces with solid-state lasers and in particular to enlarge the focus diameter to create wider cutting kerfs and improve the ejection of the molten metal.
For example, WO2011124671A1, WO2013000942A1, WO2014060091A1, US20180188544A1 or WO2018104575A1 has thus described influencing the beam quality and thus the focusability of a solid-state laser beam by coupling the beam into different cores of a multicore fiber in order to be able to cut different workpieces, in particular workpieces having different thicknesses.
In addition, DE60206184T2 or JP2000005892A has proposed to split the laser beam during laser cutting with the aid of transmissive or reflective optical elements into a plurality of partial beams that are focused at a plurality of focus points in the workpiece with an offset in the beam propagation direction. The ability to cut workpieces that are as thick as possible is likewise an objective.
In an embodiment, the present disclosure provides a method for laser cutting a workpiece having a thickness of less than 6 mm that includes the steps of directing a first laser beam, a second laser beam, and a gas jet at an entrance surface of the workpiece such that the first and second laser beams at least partially overlap one another on the workpiece. The first laser beam has a smaller focus diameter than the second laser beam, a beam parameter product of the first laser beam is at most 5 mm*mrad, and a power proportion of the second laser beam of a total laser power is less than 20%. A cutting kerf with a broken cutting edge is formed on the entrance surface of the workpiece.
Subject matter of the present disclosure will be described in even greater detail below based on the exemplary figures. All features described and/or illustrated herein can be used alone or combined in different combinations. The features and advantages of various embodiments will become apparent by reading the following detailed description with reference to the attached drawings, which illustrate the following:
It is an aspect of the present invention to specify a laser cutting method for thin workpieces having a thickness of less than 6 mm, in which high cutting speeds and good cutting quality are combined. It is furthermore an aspect of the present invention to specify a laser cutting apparatus for efficiently laser cutting workpieces having a thickness of less than 6 mm with a good cutting quality, which is suitable in particular for cutting three-dimensionally shaped metal sheets.
According to an aspect of the invention, a method for laser cutting a workpiece having a thickness of less than 6 mm is provided. Workpieces having such a thickness are frequently cut on 3D laser cutting apparatuses and used, for example, in car-body construction. The workpiece is preferably cut along a three-dimensionally extending cutting line. Laser cutting is preferably effected by laser fusion cutting. In laser fusion cutting, the material of the workpiece is melted to form a cutting kerf and is evacuated in liquid form from the cutting kerf. The workpiece can be a metal sheet, in particular a three-dimensionally shaped metal sheet. The workpiece preferably consists of a metallic and/or electrically conductive material. The method according to the invention is preferably carried out with a laser cutting apparatus according to the invention, which is described below.
In the laser cutting method according to an aspect of the invention, a first laser beam, a second laser beam, and a gas jet are directed at an entrance surface of the workpiece. The two laser beams and the gas jet cause material to melt and be removed from the workpiece, forming a cutting kerf. The entrance surface is the surface of the workpiece on which the beams and the jet are incident. Once the cutting kerf has formed, portions of the beams and the jet typically emerge from the workpiece at the opposite exit surface. The first and the second laser beam are typically formed in each case by a single laser beam. However, alternatively, the first and/or in particular the second laser beam can consist of a plurality of partial beams in each case. The two laser beams can be produced using a common laser light source and be separated from one another using a beam splitter. Alternatively, each of the two laser beams can be produced using a separate laser light source. The cutting gas in the gas jet which has been directed at the entrance surface and injected into the cutting kerf can be nitrogen or compressed air, for example. In special cases, the cutting gas can also be argon.
The laser beams at least partially overlap one another on the workpiece. In other words, the two laser beams simultaneously cover a common region on the surface or in the volume of the workpiece or in the cutting kerf in each case. The first laser beam preferably extends in the region of the workpiece entirely inside the second laser beam. In particular, the two laser beams can be overlaid to form one overall laser beam.
The first laser beam has a smaller focus diameter than the second laser beam. The beam parameter product of the first laser beam is, according to an aspect of the invention, at most 5 mm*mrad. With preference, the beam parameter product of the first laser beam is at most 3 mm*mrad and with particular preference at most 2 mm*mrad. The high beam quality of the first laser beam makes particularly high cutting speeds possible. In other words, if a beam parameter product of the first laser beam is small, i.e. with a high beam quality, the productivity of the method according to an aspect of the invention can be increased. The beam parameter product is defined as the product of the half angle of the laser beam in the far field and the radius of the laser beam at its thinnest point, i.e. half the focus diameter.
According to an aspect of the invention, provision is made for a power proportion of the second laser beam of the total laser power to be less than 20%. The total laser power is the sum of the laser powers of the first and the second laser beam. In other words, the power proportion of the first laser beam of the total laser power is at least 80%. The power proportion of the second laser beam of the total laser power is greater than zero. Typically, the power proportion of the second laser beam of the total laser power is at least 2%, preferably at least 3%. It has been found according to an aspect of the invention that in the case of thin workpieces having a thickness of less than 6 mm, high beam quality and a small focus diameter of the actual cutting beam (the first laser beam) enable the cutting speed (and consequently the productivity) to be increased while at the same time attaining a good quality of the cutting flanks at the cutting kerf when a specific part of the total laser power is focused onto the workpiece with a larger diameter (i.e. over the second laser beam). The total laser power can be at least 1 kW, preferably at least 2 kW.
Owing to the lower-power second laser beam surrounding the first laser beam (the actual cutting beam), the coupling efficiency of the cutting gas from the gas jet into the cutting kerf is improved. The method parameters are selected according to an aspect of the invention such that the cutting kerf is geometrically shaped such that conditions which are favorable in terms of flow for the cutting gas arise. According to an aspect of the invention, the cutting kerf with a broken cutting edge is formed for this purpose on the entrance surface of the workpiece. A broken cutting edge is understood to mean in particular a cutting edge that has an ablated area, i.e. a rounded or beveled cutting edge. The common intensity profile of the overlapping laser beams is designed such that the cutting kerf at the entrance surface is formed to be funnel-shaped. The funnel forms a lead-in radius or a lead-in bevel at the cutting flanks of the cutting kerf. The funnel makes it possible for the cutting gas to be able to flow into the cutting kerf with minor resistance. The pressure loss due to impacts and turbulence is significantly lower at the broken cutting edge than at a rectangular, right-angled (sharp) edge.
The cutting edge is preferably rounded. A radius of the cutting edge can be at least 20 μm, preferably at least 25 μm, and/or at most 100 μm, preferably at most 60 μm, with particular preference at most 35 μm. With very particular preference, the radius is 30 μm. These values for the radius bring about particularly advantageous conditions for the inflow of the cutting gas.
The method parameters are selected such that both the highest possible cutting speed (productivity) and also a good cutting quality are achieved. On the one hand, the power of the actual cutting beam (the first laser beam) having a smaller beam diameter and a high beam quality should be large enough to achieve a high cutting speed. On the other hand, the power of the partial beam having a greater beam diameter (the second laser beam) must be sufficiently high for the ablated area to form at the cutting edge of the cutting kerf. The power proportion of the outer, second laser beam is for this purpose advantageously selected depending on the thickness of the workpiece.
The thickness of the workpiece can be less than 5 mm and preferably more than 3 mm. In particular, the thickness can be 4 mm. The power proportion of the second laser beam of the total laser power is then preferably less than 15%.
The thickness of the workpiece can be less than 3 mm and preferably more than 1 mm. In particular, the thickness can be 2 mm. The power proportion of the second laser beam of the total laser power is then preferably less than 7%, in particular 5%.
The previously mentioned values help to bring about a good compromise between expanding the cutting kerf lead-in (due to the ablated area of the cutting edge at the entrance surface) and the highest possible productivity, i.e. cutting speed.
The focus point of the first laser beam can lie upstream of the focus point of the second laser beam in the propagation direction of the laser beams. The focus point of the first laser beam can lie within the workpiece, preferably in the half of the workpiece that is closer to the entrance surface, or outside the workpiece. The focus point of the second laser beam then lies deeper inside the workpiece, or closer to the entrance surface. The focus point of the (high-power) first laser beam preferably lies in the region of the workpiece surface. In particular, a distance of the focus point of the first laser beam from the entrance surface can be less than 30%, preferably less than 15%, of the thickness of the workpiece. A distance between the focus points of the two laser beams is preferably at most 2 mm, in particular at most 1 mm, and typically between 0.5 and 0.7 mm.
A distance of the focus point of the second laser beam from the entrance surface of the workpiece can be at most twice the Rayleigh length of the second laser beam. The Rayleigh length is defined as the quotient of the product of the refractive index of the propagation medium, the circle number pi and the square of the radius of the laser beam in the focus point as the dividend and the vacuum wavelength of the laser light as the divisor.
The focus diameter of the second laser beam can be at least twice, preferably at least three times, and/or at most five times, preferably at most four times, the focus diameter of the first laser beam. In particular, the focus diameter of the first laser beam can be at least 50 μm, preferably at least 80 μm, and/or at most 300 μm, preferably at most 150 μm. These value ranges have proven useful for different workpiece thicknesses of up to 6 mm.
The propagation axes of the two laser beams can be inclined with respect to one another or preferably be parallel to one another. Advantageously, the propagation axes coincide.
The divergences of the first and second laser beam in the far field can be the same or differ by at most ΔΘ=100 mrad. This enables a simple design of the optical system for guiding and focusing the laser beams, which contributes to the process reliability of the method.
The two laser beams can be overlaid eccentrically with respect to one another. However, the two laser beams are advantageously overlaid concentrically with respect to one another. In this way, it is possible to cut in all directions, without the orientation of the two laser beams needing to be adapted to the cutting direction, for example by rotating an optical unit in a cutting head.
Provision may be made for the two laser beams to emerge from a multicore fiber having a first fiber core for the first laser beam and a second fiber core for the second laser beam. The multicore fiber can have fibers running parallel to one another. Preferably, the second fiber core surrounds the first fiber core. In other words, the first fiber core is arranged radially inside the second fiber core. The second fiber core is therefore embodied in the form of a ring fiber. In particular, the first and the second fiber cores can be concentric with respect to each other.
The first fiber core, from which the first laser beam emerges, can have a diameter of at most 100 μm, preferably at most 50 μm. The second fiber core, from which the second laser beam emerges, can have a diameter of at most 300 μm, preferably at most 200 μm.
The gas jet of the cutting gas can emerge from a conical nozzle having a round or oval opening diameter, a secondary flow nozzle or a de Laval nozzle. A gas pressure, in particular a dynamic gas pressure, of the gas flow after it emerges from the nozzle can be at least 16 bar, preferably at least 18 bar, and/or at most 24 bar, preferably at most 22 bar. With such a gas pressure, the material of the workpiece can be reliably evacuated from the cutting kerf, in particular without burrs forming on the exit surface.
A laser cutting apparatus for laser cutting an, in particular three-dimensionally shaped, metal-sheet-type workpiece along an, in particular three-dimensional, cutting line furthermore falls within the scope of the present invention. The laser cutting apparatus is preferably a laser fusion cutting apparatus for laser fusion cutting. The laser cutting apparatus is advantageously configured for carrying out the laser cutting method according to an aspect of the invention, which was described above. In particular, substantive features which are described above can be provided in the laser cutting apparatus according to an aspect of the invention. The laser cutting apparatus can be configured to produce the first laser beam, the second laser beam, and/or the gas jet with above-described parameters and to direct them at the workpiece in the manner described above.
The laser cutting apparatus has a laser light source device for overlaying a first laser beam and a second laser beam in a cutting zone. The first laser beam has a smaller beam diameter and a smaller focus diameter than the second laser beam. The beam parameter product of the first laser beam is at most 5 mm*mrad, preferably at most 3 mm*mrad. A power proportion of the second laser beam of the total laser power is less than 20%. The laser light source device can have an optical unit for focusing the two laser beams in the cutting zone.
The laser cutting apparatus furthermore has a nozzle for directing a gas jet at the cutting zone. The gas jet provides cutting gas, for example nitrogen, compressed air or argon, for evacuating material of the workpiece from the cutting kerf that forms during laser cutting. The two laser beams typically exit through the nozzle.
The laser cutting apparatus furthermore has a movement device for moving the cutting zone relative to the workpiece along the three-dimensional cutting line. The laser cutting apparatus can have a workpiece holder that is arranged fixedly at the laser cutting apparatus, in particular at a machine bed of the laser cutting apparatus. An optical unit of the laser light source device or the entire laser light source device and the nozzle can be translationally and/or rotationally displaceable or rotatable, in particular relative to the machine bed. Alternatively, the workpiece holder can be movably arranged at a machine bed of the laser cutting apparatus. The optical unit or the laser light source device and the nozzle can then be arranged fixedly at the laser cutting apparatus. It is also conceivable to create a few degrees of freedom of the relative movement by way of a movability of the workpiece holder, for example in one or more translational directions, and other degrees of freedom by way of a movability of the optical unit or of the laser light source device and the nozzle, in particular by way of a rotability about one or more axes.
Further features and advantages of the invention are evident from the description and the drawing. According to the invention, the features mentioned above and those still to be further presented can be used in each case individually or together in any desired expedient combinations. The embodiments shown and described should not be understood as an exhaustive enumeration, but rather are of illustrative character for outlining the invention.
In order to produce the cutting kerf 12 in the workpiece 14, a first laser beam 18, a second laser beam 20, and a gas jet 22 are directed at an entrance surface 24 of the workpiece 14. The two laser beams 18, 20 and typically also the gas jet 22 in this case overlap one another in a cutting zone 26. During laser fusion cutting, the material of the workpiece 14 in the cutting zone 26 is liquefied and evacuated by the gas jet 22 while forming the cutting kerf 12.
The procedure in principle in the laser cutting method is illustrated in the flowchart of
The two laser beams 18, 20 are produced by a laser light source device 28; see
The multicore fiber 36 has a first fiber core 40 for the first laser beam 18 and a second fiber core 42 for the second laser beam 20; see also
A beam diameter 50 of the first laser beam 18 in the region of the workpiece 14 to be cut is smaller than a beam diameter 52 of the second laser beam 20. In particular, a focus diameter 54 of the first laser beam 18 is smaller than a focus diameter 56 of the second laser beam 20. The focus diameter 56 of the second laser beam 20 can be 3.5 times the size of the focus diameter 54 of the first laser beam 18. The beam parameter product of the first laser beam 18 is less than 5 mm*mrad, in the present case for example 2 mm*mrad.
A power proportion of the second laser beam 20 of the total laser power (the sum of the laser powers of the two laser beams 18, 20) is less than 20%. At a thickness 16 of the workpiece 14 of 2 mm, the power proportion of the second laser beam 20 can be, for example, 5%.
The above-described refinement of the laser cutting method causes cutting edges 58 of the cutting kerf 12 to be broken at the entrance surface 24; see
By contrast, cutting edges 58′ of a cutting kerf 12′ have sharp edges at an entrance surface 24′ of a workpiece 14′ in laser cutting methods according to the prior art; see
In order to move the cutting kerf 12 on along an, in particular three-dimensional, cutting line, the cutting zone 26 is moved in relation to the workpiece 14. The laser cutting apparatus 10 can for this purpose have a movement device 66; see
A focus point 72 of the first laser beam 18 can be offset along the propagation axis 48 with respect to a focus point 74 of the second laser beam 20. The focus point 72 of the first laser beam 18 lies here upstream of the focus point 74 of the second laser beam 20 in the propagation direction of the laser beams 18, 20. A distance 76 between the focus points 72, 74 along the propagation axis 48 can be, for example, 0.7 mm.
The second focus point 74 and preferably also the first focus point 72 can lie inside the workpiece 14, that is to say beyond the entrance surface 24 in the propagation direction of the laser beams 18, 20. A distance 78 of the first focus point 72 from the entrance surface 24 can be, for example, a quarter of the thickness 16 of the workpiece 14. A distance 80 of the second focus point 74 from the entrance surface 24 can be less than twice, for example 1.5 times, the Rayleigh length of the second laser beam 20.
The further parameters of the laser cutting apparatus 10 of
A movement unit 66 of the laser cutting apparatus 10 of
Experiments have further shown that workpieces having a thickness 16 of less than 6 mm with a small focus diameter 54 of 100 μm of the first laser beam 18 can be cut more than 30% faster than with a focus diameter 54 of 150 μm, specifically at up to 24 m/min
While subject matter of the present disclosure has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive. Any statement made herein characterizing the invention is also to be considered illustrative or exemplary and not restrictive as the invention is defined by the claims. It will be understood that changes and modifications may be made, by those of ordinary skill in the art, within the scope of the following claims, which may include any combination of features from different embodiments described above.
The terms used in the claims should be construed to have the broadest reasonable interpretation consistent with the foregoing description. For example, the use of the article “a” or “the” in introducing an element should not be interpreted as being exclusive of a plurality of elements. Likewise, the recitation of “or” should be interpreted as being inclusive, such that the recitation of “A or B” is not exclusive of “A and B,” unless it is clear from the context or the foregoing description that only one of A and B is intended. Further, the recitation of “at least one of A, B and C” should be interpreted as one or more of a group of elements consisting of A, B and C, and should not be interpreted as requiring at least one of each of the listed elements A, B and C, regardless of whether A, B and C are related as categories or otherwise. Moreover, the recitation of “A, B and/or C” or “at least one of A, B or C” should be interpreted as including any singular entity from the listed elements, e.g., A, any subset from the listed elements, e.g., A and B, or the entire list of elements A, B and C.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2020 205 948.9 | May 2020 | DE | national |
This application is a continuation of International Application No. PCT/EP2021/062440 (WO 2021/228829 A1), filed on May 11, 2021, and claims benefit to German Patent Application No. DE 10 2020 205 948.9, filed on May 12, 2020. The aforementioned applications are hereby incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2021/062440 | May 2021 | US |
| Child | 18053384 | US |
