Method for laser cutting of metal parts

Abstract

A method for laser cutting of metal parts (T) is described, comprising the operations of emitting onto a surface (S) of the part (T) to be processed, through an emission nozzle (14) of a working head (12) of a laser cutting machine (M), a dual-beam laser having an internal laser beam (Bi) and an external laser beam (Be) arranged around the internal laser beam (Bi), and at the same time moving the working head (12) with respect to the part (T) so as to move the hitting point of the laser on the surface (S) of the part (T) along at least one given cutting line, wherein the internal laser beam (Bi) is generated with process parameters such as to allow said beam to perform a cutting operation on the part (T), while the external laser beam (Be) is generated with process parameters different from those of the internal laser beam (Bi) so as to allow said beam to perform a cleaning or homogenization operation on the surface (S) of the part (T), consisting in the removal of a surface layer, of a thickness not exceeding 100 micrometers, of the same metal material of the part (T).

Description

[0001] METHOD FOR LASER CUTTING OF METAL PARTS

[0002] Technical field of the invention

[0003] The present invention relates in general to a method for laser cutting of metal parts, and more particularly to a method in which, prior to the laser cutting operation, a cleaning or homogenization operation is carried out, still using a laser beam, onto the surface of the part to be cut with the aim of improving both the quality and the monitoring process of the laser cutting method that is subsequently carried out.

[0004] State of the art

[0005] In the field of machines for laser cutting of metal parts, for example, though not exclusively, for laser cutting of tubes and profiles, it is well known that the quality of the laser cutting is influenced by the surface state of the material to be cut.

[0006] For example, as reported in Charles L. Caristan, "Laser Cutting Guide for Manufacturing", Dearborn-Michigan, Society of Manufacturing Engineers, (2004), oxide crusts, rolling residues and the presence of oil on the surface of a metal workpiece will adversely affect the quality of the cut performed by means of an oxygen-assisted laser beam. Even if an inert gas is used as an assist gas, metal oxides might affect the cutting process, reducing reliability and forcing the machine operator to make corrections to the technological parameters, for example, to reduce the cutting speed with a consequent reduction in productivity. This is also due to the fact that these crusts and residues have higher hardness and higher melting point than the base metal of the workpiece to be cut, which makes it difficult to achieve a satisfactory cutting quality.

[0007] In the event that a sand-blasting process is carried out to remove the surface oxide layer, the ceramic particles used for sand-blasting must also be removed from the surface of the workpiece, as these particles have very different physical characteristics with respect to those of the base metal material, as explained above for the presence of oxide crusts and rolling residues. Furthermore, such ceramic particles might distort the measurement, typically based on the capacitive effect, of the distance between the nozzle of the cutting head and the material of the part to be processed, thus leading to problems in the cutting process. In addition, parts made of metal material, e.g. steel, often undergo galvanising or painting processes or are coated with a plastic film in order to protect them from corrosion. This results in the presence of a surface layer of a material that is not homogenous with respect to the base material, which adversely affects the quality of laser cutting. In fact, similarly to what has been explained above with reference to the presence of ceramic particles, the presence of a surface layer of a material that is not homogenous with respect to the base material leads to errors in the measurement of the distance between the nozzle of the cutting head and the material of the part to be processed.

[0008] The quality of laser cutting can therefore be improved by performing, prior to the actual laser cutting operation, a preliminary cleaning or homogenization operation onto the surface of the part to be cut, which removes the surface layer of material other than the base material of the part to be cut, for example a surface layer formed by a plastic film, oil or paint or, alternatively, an oxidised surface layer, so that the subsequent laser cutting operation is carried out on a material with homogeneous chemical-physical characteristics.

[0009] Furthermore, removing, prior to the cutting operation, a surface layer of the metal material of which the part to be cut is made allows to improve monitoring of the cutting process. Typically, monitoring of the laser cutting process is carried out with the use of cameras or photodiodes that analyse the retroreflection of the laser beam used for cutting. A possible presence of surface defects on the part to be cut and / or a possible non-homogeneity in the reflectivity properties of the surface of the part to be cut randomly influences the signal provided by the cameras or photodiodes to the control unit that manages the operation of the cutting machine, making the analysis of the process more difficult and / or introducing errors in the estimation of the process state. Monitoring of the cutting process is therefore less reliable and accurate, with further negative consequences on the quality of the cut.

[0010] It is known to carry out a preliminary cleaning or homogenization operation onto the surface of the part to be cut using a laser beam. This preliminary operation can be carried out by introducing additional hardware into the laser cutting machine (pulsed laser source and scanning head) or, more commonly, by using the same laser source and the same cutting head but with process parameters different from those used for cutting (low power and duty cycle, for example 500W and 10%, respectively).

[0011] While the preliminary cleaning or homogenization operation is advantageous in terms of improvement in the quality and monitoring of the subsequent laser cutting process, it clearly entails a longer process time, since the time required for this preliminary operation is added to the time required for the cutting operation.

[0012] DE102011078178B4 discloses a method for laser cutting of metal sheets coated with a protective film, using a cutting head configured to emit an internal laser beam and an external laser beam arranged around the internal laser beam, wherein the internal laser beam is generated with process parameters such that this beam is able to perform a cutting operation on the metal sheet, while the external laser beam is generated with process parameters different from those of the internal laser beam in order to vaporize the protective film, without affecting the underlying sheet material, prior to the cutting operation. This allows both the vaporization of the protective film and the cutting of the metal sheet to be performed in a single step, thereby reducing the overall time required to perform the cutting process. This prior art document teaches to apply the cutting method with dual laser beam only to metal sheets coated with a protective film, with the external laser beam being used exclusively to vaporize the protective film applied to the surface of the sheet.

[0013] Summary of the invention

[0014] It is an object of the present invention to provide a method that allows to overcome the drawbacks of the prior art discussed above, in particular, that allows to improve the cutting quality and to make the monitoring of the cutting process more reliable and accurate, without thereby lengthening the time required for cutting.

[0015] This and other objects are fully achieved according to the present invention by a laser cutting method as defined in the attached independent claim 1 .

[0016] Advantageous modes for implementing the method according to the invention are specified in the dependent claims, whose subject-matter is to be understood as forming part of the following description.

[0017] In summary, the invention is based on the idea of using a laser with an internal laser beam and an external laser beam arranged around the internal laser beam, and of using the internal laser beam to perform the actual cutting operation and the external laser beam to perform the surface cleaning or homogenization operation, thus adopting for the internal laser beam appropriate process parameters suitable for the cutting operation and for the external laser beam appropriate process parameters such as to allow this laser beam to remove from the surface of the part to be cut a layer of the same metal material of that part with a thickness not exceeding 100 micrometres. Thanks to such a solution, as the working head is moved along the surface of the metal part to be cut (whereby the term "moved" means that a relative movement of the working head with respect to the part to be cut is imposed, which can be obtained either by virtue of the movement of the working head alone or by virtue of the movement of the part to be cut alone, or even by combined movement of the working head and the part to be cut), the external laser beam operates on the surface of the part to be cut, making this surface homogenous and clean by removing a very thin surface layer of material, before the internal laser beam, and therefore before the actual cutting operation is carried out by the internal laser beam. In this way, the surface cleaning or homogenization operation is carried out while the cutting operation is in progress, without therefore resulting in a lengthening of the process time. Furthermore, thanks to the fact that the surface of the part to be cut is made homogenous and clean before cutting, the monitoring of the cutting process by means of cameras or photodiodes is more reliable and accurate, as it is not adversely affected (or significantly less so compared to the prior art) by any surface defects on the part to be cut and / or any inhomogeneity in the reflectivity properties of the surface of the part to be cut.

[0018] Additionally, since the surface cleaning or homogenization operation is performed by the external laser beam, which is arranged around the internal beam, this operation is always performed at any point on the surface of the part to be cut prior to the cutting operation, regardless of the direction of relative movement of the working head with respect to the part to be cut.

[0019] A further advantage of the present invention is the possibility of increasing the cutting speed, by virtue of the fact that the surface of the material is made homogenous by performing the preliminary operation of surface cleaning or homogenization.

[0020] The external laser beam used for carrying out the surface cleaning or homogenization operation will have process parameters suitably chosen to remove from the part to be cut only a surface layer of material with a minimum thickness (namely, as explained above, a thickness of no more than 100 micrometres, in particular a thickness between 20 and 60 micrometres), in particular a low power (for example, between 350W and 1000W, in particular between 500W and 1000W) and a low duty cycle (for example, less than 25%, in particular between 10% and 20%).

[0021] Preferably, the internal laser beam and the external laser beam are generated and launched into two different optical fibres, in particular into two coaxial optical fibres, which prevents (constructive or destructive) interactions between the two laser beams along the optical path.

[0022] Brief description of the drawings Further features and advantages of the present invention will be apparent from the following detailed description, given purely by way of non-limiting example with reference to the accompanying drawings, wherein:

[0023] - Figure 1 is a schematic illustration of an apparatus for laser cutting of metal parts, such as tubes and profiles, configured to perform the cutting method according to the present invention;

[0024] - Figure 2 is a schematic illustration of the laser generation and transport system of the machine of Figure 1 ;

[0025] - Figure 3 shows the cross-section of a dual-beam laser transport fibre used by the machine of Figure 1 to perform cutting operations on a metal part; and

[0026] - Figures 4 and 5 are schematic views showing an example of how the cutting method according to the present invention is performed.

[0027] Detailed description

[0028] Referring first to Figure 1 , a machine for laser cutting of metal parts, for example (though not exclusively) tubes and profiles with cross-sections of any shape and size, such as circular tubes, square tubes, rectangular tubes, C-profiles, T-profiles, IPE-profiles, HEA- profiles, etc., is generally indicated with M.

[0029] In the example illustrated in Figure 1 , the machine M is a machine designed to work with tubes and profiles, and therefore for convenience in the following description reference will be made to a tube as the metal part to be cut by means of the machine M. As mentioned, however, the invention is not limited to laser cutting of tubes and profiles, and therefore the machine implementing the cutting method of the present invention may have a different configuration from the one illustrated herein so as to be suitable for working on parts other than tubes and profiles, such as, for example, sheet metal parts. The machine M comprises, in a per-se-known manner, a base 10, a working head 12 configured to perform cutting operations on a tube T by means of a focused laser beam emitted through an emission nozzle 14, and a feeding device 16 configured to move the tube T forward along a feed axis (or working axis) x towards the working head 12. In the illustrated embodiment, the machine M further comprises a supporting and guiding device 18, which is arranged upstream of the working head 12 and is configured to support and guide the tube T as the latter is moved forward by the feeding device 16 towards the working head 12. The supporting and guiding device 18 may however not be provided. The working head 12 is movable relative to the base 10 in a transverse plane, i.e., in a plane perpendicular to the feed axis x (vertical plane, assuming that the feed axis x is directed horizontally). In particular, the working head 12 is movable in the transverse plane at least along a horizontal direction y, preferably both in the horizontal direction y and in a vertical direction z. Furthermore, as provided for in the illustrated example, the working head 12 is preferably (although not necessarily) supported so as to be able to tilt about a transverse tilt axis t (or, according to an embodiment not shown, about two tilt axes orthogonal to each other). The working head 12 may also be moved in a direction parallel to the feed axis x.

[0030] The feeding device 16 is preferably arranged to control not only a translational movement of the tube T along the feed axis x (feed movement), but also a rotational movement of the tube T around that axis. In the case of a laser tube cutting machine, the combination of the degrees of freedom of movement of the working head 12 (translation along the horizontal direction y and translation along the vertical direction z, as well as possible rotation around the tilt axis t and / or translation along the direction of the feed axis x) and the degrees of freedom of movement of the tube T (translation along the direction of the feed axis x and rotation around the feed axis x) allows a cut to be made on the wall of the tube T along any cutting line.

[0031] The laser used by the working head 12 to perform cutting operations on the tube T is a dual-beam laser comprising an internal laser beam Bi and an external laser beam Be of annular shape, arranged around the internal laser beam Bi, concentrically with the latter. Between the two laser beams Bi and Be there may be an inactive annular zone, in which there is no laser radiation or in which the laser radiation is in any case too low to cause removal of material from the surface of the part to be processed.

[0032] The machine M further comprises a monitoring system (not shown in the drawings, but anyway of per-se-known type) configured to monitor the cutting process in real time. The monitoring system comprises, for example, cameras or photodiodes designed to analyse the retroreflection of the laser beam used for cutting, i.e. to acquire signals relating to the laser beam that is reflected by the surface of the part to be cut during the cutting process. Referring to Figures 2 and 3, the internal laser beam Bi and the external laser beam Be are generated independently of each other by a first laser source 20 and a second laser source 22, respectively, and transmitted via a first optical fibre cable 24 and a second optical fibre cable 26, respectively, to a combiner device 28. In the combiner device 28, the internal laser beam Bi and the external laser beam Be are combined to form a dualbeam laser, which is transmitted via a third coaxial optical fibre cable 30 to the working head 12, where it is collimated by a dioptric collimation system 32 and reflected by a catoptric system 34 before being focused via a focusing optical system 36 along an optical axis of propagation incident on a surface S of the tube T being processed, passing through the emission nozzle 14.

[0033] The cutting method is performed by simultaneously emitting both the internal laser beam Bi and the external laser beam Be onto the surface S of the tube T, with process parameters suitably selected for the two beams. More specifically, the internal laser beam Bi is set to perform the actual cutting of the material of the tube T, while the external laser beam Be is set to perform a surface cleaning or homogenization operation, i.e. to remove a layer of metal material of the tube T with a thickness not exceeding 100 micrometres, in particular (although not necessarily) between 20 and 60 micrometres. Therefore, while the process parameters used for the internal laser beam Bi are chosen (in a per-se-known manner) to perform the actual cutting of the metal material of the tube T, thus taking into account in particular the type of metal material and the thickness of the material to be cut, as well as the type of assist gas used for cutting, the process parameters used for the external laser beam Be are chosen to cause the removal of only the outermost layer of material of the tube T. The internal laser beam Bi will typically have a high power and duty cycle, while the external laser beam Be will have a low power, for example between 350Wand 1000W, in particular between 500W and 1000W, and a low duty cycle, for example less than 25%, in particular between 10% and 20% (assuming the use of a continuous laser source and assuming the use of the same optical path as that of the cutting laser). Advantageously, the process parameters for the external laser beam Be will be chosen depending on the metal material of the tube T and the thickness of the material to be removed, but independently of the thickness of the material to be cut. Of course, if residues of material other than the material of the tube T are present on the surface of the same tube, such as oil or paint residues or oxides, the cleaning or homogenization operation performed by means of the external laser beam Be will also remove these residues, along with a surface layer of the metal material of the tube.

[0034] As shown in Figures 4 and 5, thanks to such a solution, as the working head 12 is moved along the surface S of the tube T (according to the example illustrated therein, towards the right along a substantially straight path), either as a result of the movement of the working head 12 alone or as a result of the movement of the tube T alone, or even as a result of the combined movement of the working head 12 and the tube T, the external laser beam Be acts on the surface S before the internal laser beam Bi. Therefore, before the actual cutting operation is performed by the internal laser beam Bi, a surface cleaning or homogenization operation is carried out by the external laser beam Be, with removal of a surface layer, with a maximum thickness of 100 micrometres, of the metal material of the tube T. In this way, the surface cleaning or homogenization operation is carried out while the cutting operation is in progress, without therefore resulting in a lengthening of the process time. Since the external laser beam Be completely surrounds the internal laser beam Bi, the cleaning or surface homogenization operation by the external laser beam Be is performed at any point on the surface S prior to the cutting operation regardless of the direction of relative movement of the working head 12 with respect to the tube T. In the event that defects D are present on the surface S of the tube to be cut, the cleaning or homogenization operation by the external laser beam Be also allows, as mentioned above, to remove such defects D.

[0035] Making the surface S of tube T to be cut homogenous before carrying out the actual cutting operation allows to increase the cutting speed, and therefore to speed up the process. Furthermore, making the surface S of the tube T to be cut homogenous before carrying out the actual cutting operation allows to improve the monitoring of the method by analysing the retroreflection of the laser beam (more precisely, of the internal laser beam Bi), since the signals acquired by the cameras or photodiodes of the monitoring system are not influenced by possible surface defects or inhomogeneities of the tube to be cut.

[0036] The present invention has been described herein with reference to possible modes of implementation. It is to be understood that other modes of implementation may be envisaged, which share the same inventive core with those described herein, as defined in the appended claims.

Claims

CLAIMS1. Method for laser cutting of a metal part (T), comprising the operations of emitting onto a surface (S) of said part (T), through an emission nozzle (14) of a working head (12) of a laser cutting machine (M), a laser having an internal laser beam (Bi) and an external laser beam (Be) arranged around the internal laser beam (Bi), and at the same time moving the working head (12) relative to the part (T) so as to move the hitting point of the laser on the surface (S) of the part (T) along at least one given cutting line, wherein the internal laser beam (Bi) is generated with such process parameters as to enable that beam to perform a cutting operation on the part (T), while the external laser beam (Be) is generated with process parameters different from those of the internal laser beam (Bi) so as to remove from the surface (S) of the part (T), prior to the cutting operation by means of the internal laser beam (Bi), a layer of the same metal material as that of said part (T) with a thickness not exceeding 100 micrometres.

2. Method according to claim 1 , wherein the layer of metal material removed from the part (T) by the external laser beam (Be) has a thickness between 20 and 60 micrometres.

3. Method according to claim 1 or claim 2, wherein the external laser beam (Be) is an annular shaped laser beam, coaxial to the internal laser beam (Bi).

4. Method according to any one of the preceding claims, wherein the external laser beam (Be) has a lower power than the internal laser beam (Bi), in particular a power not exceeding 1000W, preferably between 500W and 1000W.

5. Method according to any one of the preceding claims, wherein the external laser beam (Be) has a duty cycle of not more than 25%, preferably between 10% and 20%.

6. Method according to any one of the preceding claims, wherein the internal laser beam (Bi) and the external laser beam (Be) are each generated by a respective laser source (20, 22), independently of each other.

7. Method according to any one of the preceding claims, wherein the internal laser beam (Bi) and the external laser beam (Be) are transmitted to the working head (12) via a coaxial optical fibre cable (30), so as to avoid interactions between said beams.

8. Method according to any one of the preceding claims, further comprising the operation of monitoring the cutting method by analysis of the retroreflection of the internal laser beam (Bi) with which the cutting operation is performed.

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