LASER WELDING METHOD AND DEVICE

Abstract

The invention relates to a method for laser welding workpieces (W), a laser beam (L) directed onto a workpiece surface having such a radiation intensity that the workpiece material of the at least one workpiece (W) to be welded is melted in the region of the laser focus (F), a vapor capillary (D) which is at least partly surrounded by a molten bath (S) forming in the region of the laser focus (F). The laser beam (L) is moved relative to the workpiece surface in a direction of advance (V) in order to produce a weld seam. According to the invention, the molten bath (S) is subjected to mechanical stress by directing a gas stream (G) onto the workpiece surface for the purpose of stabilization during welding. The invention further relates to a device (1) designed for carrying out said method.

Claims

1. A method for laser welding of workpieces (W), wherein a laser beam (L) oriented onto a workpiece surface has a radiation intensity such that the workpiece material of the at least one workpiece (W) to be welded is melted in the region of a laser focus (F), wherein a vapor capillary (D) forms in the region of the laser focus (F), which is enclosed at least in sections by a liquid molten pool (S), wherein the laser beam (L) is moved in relation to the workpiece surface in a feed direction (V) to produce a weld seam, wherein the molten pool (S) is mechanically stressed for stabilization during the welding process by application of a gas flow (G) oriented onto the workpiece surface, characterized in that the laser beam (L) is moved in relation to the workpiece surface in the feed direction (V) at a feed speed (v.sub.w) to produce the weld seam and a hydrodynamic dynamic pressure (p.sub.d) of the gas flow (G) applied to the workpiece (W) is set as a function of the feed speed (v.sub.w) in such a way that the hydrodynamic dynamic pressure (p.sub.d) is at least half as much and at most four times as much as a reference dynamic pressure (p.sub.s) selected proportionally to the feed speed (v.sub.w), which is given by the relationship p.sub.s=k*v.sub.w, wherein the proportionality factor k in the SI unit system is k=7.2*10.sup.3 Pa s/m.

2. The method as claimed in claim 1, characterized in that the application of gas to the molten pool (S) is performed by means of a gas flow (G) oriented in the feed direction (V) or against the feed direction (V), wherein the flow direction of the gas flow (G) extends at an angle (α), which is less than 35°, with respect to an optical axis (O) associated with the laser beam (L).

3. The method as claimed in claim 2, characterized in that the application of gas to the molten pool (S) is performed by means of a gas flow (G) oriented in the feed direction (V), which extends at an angle (α), which is less than 10°, with respect to the optical axis (O), and/or the application of gas to the molten pool (S) is performed by means of a gas flow (G) oriented against the feed direction (V), which extends at an angle (α), which is less than 30°, with respect to the optical axis (O).

4. The method as claimed in claim 1, characterized in that the gas flow is oriented onto a region around the laser focus (F), the radius of which is at most twice a nozzle orifice diameter of a nozzle (9) providing the gas flow (G).

5. The method as claimed in claim 1, characterized in that the hydrodynamic dynamic pressure (p.sub.d) is given by density (ρ) and a flow speed (v.sub.g) of the gas by way of the relationship p.sub.d=½ρ*v.sub.g.sup.2, wherein the flow speed (v.sub.g) results according to vg=VS/A, from the quotient of a volume flow (VS) of the gas flow (G) and a flow cross section (A), through which the volume flow (VS) flows.

6. The method as claimed in claim 1, characterized in that the feed speed (v.sub.w) is greater than 5 m/min, in particular at least 6 m/min.

7. A device (1) for laser welding, which is designed to carry out a method as claimed in any one of the preceding claims, comprising a carrier for at least one workpiece (W) to be welded, a laser source and a laser optical unit (5) for generating a laser beam (L) oriented onto a workpiece surface, a gas supply (7) for producing a gas flow (G) oriented onto the at least one workpiece surface, wherein at least the laser optical unit (5) and the carrier are movably mounted in relation to one another in such a way that the laser beam (L) can be guided at least over a section along the workpiece surface in the feed direction (V), characterized in that the gas supply (7) is designed to mechanically stress a molten pool (S) formed in the region of a laser focus (F) by application of gas.

8. The device (1) as claimed in claim 7, characterized in that the gas supply (7) has at least one nozzle (9) oriented onto the workpiece surface in the feed direction (V) or against the feed direction (V) to provide the gas flow (G), wherein the nozzle (9) is aligned or can be aligned at an angle (α), which is less than 30°, with respect to an optical axis (O) of the laser optical unit (5).

9. The device (1) as claimed in claim 8, characterized in that the nozzle (9) oriented in the feed direction (V) is aligned at an angle (α), which is less than 10°, with respect to the optical axis (O) and/or the nozzle (9) oriented against the feed direction (V) is aligned or can be aligned at an angle (α), which is less than 30°, with respect to the optical axis (O).

10. The device (1) as claimed in claim 7, characterized in that the gas supply (7) has a nozzle (9), which can be aligned or is aligned coaxially to an optical axis (O) of the laser optical unit (5).

11. The device (1) as claimed in claim 10, characterized in that the nozzle (9) has a nozzle orifice surface, which delimits a flow cross section (A) and is in particular in the form of a circle or circular ring, and which is arranged coaxially to the optical axis (O) of the laser optical unit (5).

12. The device (1) as claimed in claim 7, characterized by a control unit having a control routine implemented therein for automatically setting the gas supply (7) as a function of the feed speed (v.sub.w), in particular for automatically setting a hydrodynamic dynamic pressure (p.sub.d) of the gas flow (G) provided by the gas supply (7) as a function of the feed speed (v.sub.w) according to a method as claimed in claim 1.

13. The device (1) as claimed in claim 7, characterized in that the laser source has a laser power of at least 3 kW.

14. The device (1) as claimed in claim 7, characterized in that the laser source is designed to provide laser radiation (L) having a wavelength of less than 10 μm, in particular less than 5 μm, preferably less than 2 μm, particularly preferably between 350 nm and 1300 nm.

Description

[0043] Possible exemplary embodiments of the invention are explained in greater detail hereinafter with reference to the drawings. In the figures:

[0044] FIG. 1: shows a device for laser welding having a gas supply oriented against a feed direction for mechanically stressing a molten pool in a sectional illustration;

[0045] FIG. 2: shows a device for laser welding having a gas supply oriented in the feed direction for mechanically stressing a molten pool in a sectional illustration;

[0046] FIG. 3: shows a device for laser welding having a gas supply oriented coaxially to an optical axis for mechanically stressing a molten pool in a sectional illustration.

[0047] Parts corresponding to one another are provided with the same reference signs in all figures.

[0048] FIGS. 1 and 2 schematically illustrate a first embodiment of a device 1 for laser welding, which is designed for the purpose of mechanically stressing a molten pool S formed during the welding procedure, in particular deep welding.

[0049] The device 1 has a processing head 3, which has at least one laser optical unit 5 focusing a laser beam L and a gas supply 7 having nozzle 9. A laser source (not shown in greater detail), for example, a solid-state laser or fiber laser, generates the laser beam L. An optical axis O of the laser optical unit 5 is oriented essentially perpendicularly to a workpiece surface of a workpiece W to be welded. The laser optical unit 5 orients the laser beam L onto the workpiece W, wherein the laser optical unit 5 is protected by a window 11 during the processing. The laser focus F of the laser beam L is located in the schematically illustrated example in the vicinity of the workpiece surface and generates there, due to the high intensity of the provided laser beam L, a vapor capillary D having plasma plume. The vapor capillary D is located in the molten pool S, i.e., it is enclosed by liquid molten material. The workpiece W is furthermore fixed on a carrier (not shown in greater detail), which is movably mounted relative to the processing head 3 in such a way that the workpiece W can be guided the feed direction V in relation to the provided laser beam L to produce a weld seam.

[0050] At least the gas supply 7 having nozzle 9 is rotatably mounted with respect to the optical axis O, so that it is possible to orient the gas supply 7 correspondingly, as shown in FIG. 1, to produce a gas flow G oriented against the feed direction V or, as shown in FIG. 2, to produce a gas flow G oriented in the feed direction V. The nozzle 9 is oriented in the illustrated example at an angle α of approximately 25° with respect to the optical axis O. Since the laser beam L is oriented perpendicularly on the workpiece surface, this corresponds to an application to the molten pool S at an angle α of approximately 25° with respect to a surface normal N extending perpendicularly to the workpiece surface at the location of the laser focus.

[0051] FIG. 3 shows the schematic structure of a second embodiment of a device 1 for laser welding, which is designed to mechanically stress the molten pool S formed during the welding process. The device 1 for laser welding differs structurally from the first exemplary embodiment shown in FIGS. 1 and 2 solely with respect to the geometry of the gas supply, so that reference is made to the description in this regard.

[0052] The processing head 3 of the second exemplary embodiment is designed as a coaxial head, i.e., the gas supply 7 having nozzle 9 produces a gas flow G, which extends coaxially to the optical axis O. With perpendicular alignment of the laser beam L on the workpiece surface, the gas flow G is thus applied to the molten pool G essentially in the direction of the surface normal N, i.e., at an angle α of approximately 0°.

[0053] In a method for laser welding, the laser beam L is guided along the workpiece surface in the feed direction V to locally melt the workpiece material in the region of the laser focus F. The feed speed v.sub.w along the feed direction V is in particular 1 m/min to 50 m/min, for example, 4 m/min to 24 m/min. The angle α, at which the gas flow G is incident on the molten pool S, is preferably between 0° and 35°. A nozzle orifice surface of the nozzle 9 limiting the flow cross section A of the gas flow G has, for example, a diameter of a few millimeters, in particular less than 4 mm, for example, approximately 3 mm. The nozzle orifice surface is typically spaced apart several millimeters, for example, between approximately 5 mm and 15 mm, from the welding capillary or vapor capillary D.

[0054] The application to the molten pool is to take place with a force which is suitable for stabilizing it, but expelling material at least to a noticeable extent is also to be avoided. The hydrodynamic pressure p.sub.t has proven to be a suitable parameter for the dimensioning of the gas flow G, which may be computed in simplified form from the density ρ and the flow speed v.sub.g of the outflowing gas according to p.sub.d=½ρ*v.sub.g.sup.2. The flow velocity v.sub.g can be derived in simplified form from the relationship v.sub.g=VS/A, wherein VS denotes the volume flow of the gas flow G through the flow cross section A. The volume flow VS in typically dimensioned nozzles is several liters per minute (1/min).

[0055] The gas flow for mechanically stressing the molten pool S is preferably set such that the produced hydrodynamic dynamic pressure p.sub.d is within an interval around a reference dynamic pressure p.sub.s. The gas flow is set as a function of the type of gas, nozzle orifice opening surface, and feed speed v.sub.w in such a way that the dynamic pressure p.sub.d is at least half as much as a reference dynamic pressure p.sub.s and at most four times as much as the reference dynamic pressure, i.e. (0.5*p.sub.s<p.sub.d<4*p.sub.s).

[0056] The reference dynamic pressure p.sub.s is given by p.sub.s=k*v.sub.w, wherein the proportionality factor (k) in the SI unit system is k=7.2*10.sup.3 Pa s/m.

[0057] In a specific exemplary embodiment, a stainless steel plate of the thickness 1.5 mm is welded. A fiber-guided laser provides a laser beam L of 4.5 kW. The laser optical unit 5 used has, for example, an imaging ratio of 120:300 and images a 200 μm fiber diameter on the workpiece surface, so that a laser focus having spot diameter of approximately 0.5 mm results there. At a feed speed v.sub.w of 12 m/min, argon is applied to the molten pool. The provided gas flow has a volume flow of 20 L/min, which is limited by a nozzle 9, the diameter of which is 3 mm. This corresponds to a hydrodynamic dynamic pressure p.sub.d of approximately 2 kPa, i.e., approximately 1.38 times the reference dynamic pressure p.sub.s.

[0058] The invention was described above with reference to preferred exemplary embodiments. However, it is apparent that the invention is not restricted to the specific design of the exemplary embodiments shown, rather a person of relevant skill in the art can derive variations on the basis of the description without deviating from the essential basic concept of the invention.

LIST OF REFERENCE SIGNS

[0059] 1 device [0060] 3 processing head [0061] 5 laser optical unit [0062] 7 gas supply [0063] 9 nozzle [0064] 11 window [0065] O optical axis [0066] L laser beam [0067] W workpiece [0068] S molten pool [0069] D vapor capillary [0070] V feed direction [0071] G gas flow [0072] α angle [0073] A flow cross section