Method for Laser Beam Welding of One or More Steel Sheets Made of Press-Hardenable Manganese-Boron Steel

Abstract

A method for laser beam welding of one or more steel sheets made of press-hardenable manganese-boron steel is disclosed. At least one of the steel sheets has a coating of aluminium. The laser beam welding takes place by feeding an additional wire into a melt bath generated by of a laser beam. The additional wire contains at least one austenite-stabilising alloy element. The weld seam after hot forming (press hardening) has a strength that is comparable to the base material. The laser beam is put into oscillation such that it oscillates transverse to the welding direction, wherein the oscillation frequency of the laser beam is at least 200 Hz, preferably at least 500 Hz. The method dispenses with removing the aluminum coating at the edge of the sheet-metal edges to be welded.

Claims

1. A method for laser beam welding of one or more steel sheets made of press-hardenable manganese-boron steel, wherein at least one of the steel sheets has a coating made of aluminium, comprising: feeding an additional wire into a melt bath generated by a laser beam, wherein the additional wire contains at least one austenite-stabilising alloy element, wherein the laser beam is set into oscillation such that the laser beam oscillates transverse to a welding direction, and wherein oscillation frequency of the laser beam is at least 200 Hz.

2. The method according to claim 1, wherein the one or more steel sheets are joined during laser beam welding in a butt joint or an overlap joint with a gap of less than 0.8 mm.

3. The method according to claim 1, wherein an amplitude of the oscillation of the laser beam is less than 2 mm.

4. The method according to claim 1, wherein the laser beam welding is carried out at an advance speed of more than 4 m/min.

5. The method according to claim 1, wherein the oscillation of the laser beam is carried out with a linear, circular, or polygonal oscillation profile.

6. The method according to claim 1, wherein a geometry of a weld seam is detected, and wherein at least one of the oscillation frequency and an amplitude of the oscillating laser beam is varied as a function of the detected geometry of the weld seam.

7. The method according to claim 1, wherein the additional wire has a carbon mass proportion of at least 0.1% by weight.

8. The method according to claim 1, wherein the additional wire has the following composition: 0.1 to 4.0% by weight C, 0.5 to 2.0% by weight Si, 1.0 to 2.5% by weight Mn, 0.5 to 2.0% by weight Cr+Mo, 1.0 to 4.0% by weight Ni, and remainder iron and unavoidable impurities.

9. The method according to claim 1, wherein the additional wire is heated prior to the feeding into the melt bath at least in a longitudinal section to a temperature of at least 50 C.

10. The method according to claim 1, wherein inert gas is applied to the melt bath during the laser beam welding.

11. The method according to claim 1, wherein the one or more steel sheets have a sheet thickness in the range of 0.5 to 4 mm.

12. The method according to claim 1, wherein the one or more steel sheets have at least one of a different sheet thickness and a different tensile strength.

13. The method according to claim 1, wherein the oscillation frequency is at least 500 Hz.

14. The method according to claim 2, wherein the gap is less than 0.6 mm.

15. The method according to claim 2, wherein the gap is less than 0.4 mm.

16. The method according to claim 3, wherein the amplitude of the oscillation of the laser beam is less than 1 mm.

17. The method according to claim 4, wherein the laser beam welding is carried out at an advance speed in the range of 5 to 8 m/min.

18. The method according to claim 7, wherein the additional wire has a carbon mass proportion of around at least 0.3% by weight.

19. The method according to claim 9, wherein the additional wire is heated prior to the feeding into the melt bath at least in a longitudinal section to a temperature of at least 90 C.

20. The method according to claim 11, wherein the at least one or more steel sheets have a sheet thickness in the range of 0.8 to 2.5 mm.

Description

[0029] The invention is explained in detail below on the basis of a drawing representing a plurality of exemplary embodiments, wherein:

[0030] FIG. 1 shows a schematic representation of parts of a device for carrying out the laser beam welding method according to the invention, partially in a vertical sectional view, wherein two press-hardenable steel plates of equal thickness are welded together;

[0031] FIG. 2 shows a schematic representation of parts of a device for carrying out the laser beam welding method according to the invention, partially in a vertical sectional view, wherein two press-hardenable steel plates of different thickness are welded together; and

[0032] FIG. 3 shows a perspective, schematic representation of parts of a device for carrying out the laser beam welding method according to the invention, wherein two press-hardenable steel plates in turn are welded together.

[0033] A laser beam welding device is sketched in FIG. 1, by means of which the method according to the invention can be carried out. The device comprises an underlay (not shown) on which two strips or plates 1, 2 made of steel of equal or different material qualities are arranged such that their edges to be welded together lie to one another as a butt joint. At least one of the steel sheets 1, 2 is produced from press-hardenable manganese-boron steel. The steel sheets 1, 2 are joined with a gap 3 of a few tenths of a millimetre in the butt joint (cf. FIG. 3). The gap 3 is for example less than 0.6 mm, preferably less than 0.4 mm. As far as the steel sheets 1, 2 are produced from steel of different material qualities, one steel sheet 1 or 2 for example has a relatively soft deep-drawing grade, while the other steel sheet 2 or 1 consists of higher strength steel.

[0034] The press-hardenable steel, of which at least one of the steel sheets 1, 2 to be connected to one another for example in the butt joint consists, can for example have the following chemical composition: [0035] Max. 0.45% by weight C, [0036] Max 0.40% by weight Si, [0037] Max 2.0% by weight Mn, [0038] Max 0.025% by weight P, [0039] Max 0.010% by weight S, [0040] Max 0.8% by weight Cr+Mo, [0041] Max 0.05% by weight Ti, [0042] Max 0.0050% by weight B, and [0043] Min 0.010% by weight Al, [0044] Remainder iron and unavoidable impurities.

[0045] In the delivery state, i.e. prior to a heat treatment and rapid cooling, the press-hardenable steel plates 1, 2 have a yield strength Re of preferably at least 300 MPa; their tensile strength Rm is e.g. at least 480 MPa, and their elongation at break A.sub.80 is preferably at least 10%. Following hot forming (press hardening), i.e. heating to austenitization temperature of approx. 900 to 950 C., forming at this temperature and subsequent rapid cooling, the steel plates 1, 2 have a yield strength Re of approx. 1100 MPa, a tensile strength Rm of approx. 1500 to 2000 MPa and an elongation at break A.sub.80 of approx. 5.0%.

[0046] The steel sheets 1, 2 are provided with a metallic coating 4 made of aluminium. It is preferably an AlSi coating. The metallic coating 4 is applied to the base material on both sides, for example by hot dip coating, by guiding a strip made of press-hardenable manganese-boron steel through a AlSi melt bath, blowing off excessive coating material from the strip and the coated strip then subsequently treated, in particular heated. The aluminium content of the coating 4 can be in the range of 70 to 90% by weight.

[0047] Alternatively, also only one of the steel sheets 1, 2 to be welded can have an aluminium coating 4. Furthermore, the aluminium coating 4 may, where appropriate, be applied only on one side of the steel sheet(s) 1, 2, e.g. by means of physical vapour deposition (PVD) or by means of an electrolytic coating process.

[0048] The steel sheets 1, 2 are for example substantially the same thickness in the exemplary embodiment shown in FIG. 1. The sheet thickness is for example in the range of 0.8 to 3.0 mm, wherein the thickness of the coating on the respective sheet side is less than 100 m, in particular less than 50 m.

[0049] A section of a laser beam welding head 5 is sketched above the steel sheets 1, 2, which is provided with optics to form and align a laser beam 6, in particular a focussing lens 7. The laser beam 6 is generated for example by means of an Nd:YAG laser system which delivers an output for example in the range of 5 to 6 kW.

[0050] A line 8 for feeding inert gas is assigned to the laser beam welding head 5. The discharge of the inert gas line 8 is substantially directed to the melt bath 9 generated with the laser beam 6. Pure argon or for example a mixture of argon, helium and/or carbon dioxide is preferably used as the inert gas.

[0051] In addition, a wire feeding device 10 is assigned to the laser beam welding head 5 by means of which a special additional material in the form of a wire 11 is supplied to the melt bath 9, which is also melted by the laser beam 6. The additional wire 11 is supplied to the melt bath 9 preferably in a heated state. To this end, the wire feeding device 10 is equipped with at least one heating element 12, for example a heating spiral surrounding the wire 11. Using the heating element, the additional wire 11 is preferably heated to a temperature of at least 50 C., particularly preferably to at least 90 C.

[0052] The additional wire 11 contains substantially no aluminium. It has for example the following chemical composition: [0053] 0.1% by weight C, [0054] 0.8% by weight Si, [0055] 1.8% by weight Mn, [0056] 0.35% by weight Cr, [0057] 0.6% by weight Mo, and [0058] 2.25% by weight Ni, [0059] Remainder iron and unavoidable impurities.

[0060] The additional wire 11 is supplied to the melt bath 9 generated by means of the laser beam 6 in order to reduce the mass content of the aluminium introduced into the melt bath 9 by melting the coating 4 and to homogenise the melt bath 9 or the weld seam. The additional wire 11 contains austenite-stabilising alloy elements.

[0061] The manganese content of the additional wire 11 is in this case always higher than the manganese content of the base material of the coated steel sheets 1, 2. The manganese content of the additional wire 11 is preferably approx. 0.2% by weight higher than the manganese content of the base material of the coated steel sheets 1, 2. Furthermore, it is favourable when the content of chromium and molybdenum of the additional wire 11 is higher than in the base material of the steel sheets 1, 2. The combined chromium-molybdenum content of the additional wire 11 is preferably approx. 0.2% by weight higher than the combined chromium-molybdenum content of the base material of the steel sheets 1, 2. The nickel content of the additional wire 11 is preferably in the range of 1 to 4% by weight. In addition, the additional wire 11 preferably has a carbon content of at least 0.1% by weight, particular preferably at least 0.3% by weight.

[0062] In order to achieve further homogenisation of the weld seam and to reduce the metallic notch to the base material, the laser beam 6 is set into oscillation such that it oscillates at high frequency transverse to the welding direction.

[0063] The oscillation of the laser beam 6 is indicated in FIG. 1 by the arrows 14 directed transverse to the joint. The oscillation frequency of the laser beam 6 is at least 200 Hz, preferably at least 500 Hz, particularly preferably at least 600 Hz. The oscillation of the laser beam 6 is for example caused by means of a diversion mirror (deflection mirror) 15, which is provided with an actuator 16 setting the mirror 15 into high-frequency oscillations, for example a piezo drive (piezo actuator). The diversion mirror 15 can also be advantageously configured as a focussing mirror.

[0064] The amplitude of the laser beam oscillation is preferably less than 2 mm. When joining the steel sheet plates 1, 2 with a gap 3 of a few tenths of a millimetre, e.g. a gap width in the range of 0.9 to 0.2 mm, the amplitude of the oscillation of the laser beam can for example be in the range of 1.5 to 0.5 mm. The oscillation of the laser beam 6 is carried out with a determined oscillation profile (beam figure). The actuator assigned to the diversion mirror (deflection mirror) 15 and the support of the diversion mirror 15 are preferably configured or settable such that the oscillation of the laser beam 6 has a linear, circular or polygonal oscillation profile. The circular beam figure can in this case have a circular-ring, oval or 8-shaped oscillation profile contour. The polygonal beam figure can, in contrast, in particular have a triangular, rectangular or trapezoidal oscillation profile contour. The support of the diversion mirror 15 capable of oscillating is for example implemented by means of a spring-elastic suspension and/or a fixed body joint.

[0065] The steel sheets 1, 2 are welded at an advance speed of preferably more than 4 m/min, for example at an advance speed in the range of 5 to 6 m/min, wherein either the steel sheets 1, 2 are moved by means of a movable underlay relative to the laser beam 6 or the laser beam 6 is moved by means of a robot arm relative to the steel sheets 1, 2. In this case a superimposition of the oscillation profile of the laser beam 6 with the advance movement of the steel sheets 1, 2 or the laser beam welding head 5 arises.

[0066] The embodiment sketched in FIG. 2 differs from the example shown in FIG. 1 in that the steel sheets 1, 2 have different thicknesses such that a thickness jump d is present at the butt joint. For example, the steel sheet 2 has a sheet thickness in the range of 0.8 mm to 1.2 mm, while the other steel sheet 1 has a sheet thickness in the range 1.6 mm to 3.0 mm. Moreover, the steel sheets 1, 2 to be connected to one another in the butt joint can also differ from one another in their material quality. For example, the thicker plate 1 is produced from a higher-strength steel, whereas the thinner steel plate 2 has a relatively soft deep-drawing grade. The steel sheets 1, 2 are also joined together with a gap of a few tenths of a millimetre.

[0067] The laser beam welding device used to join the steel sheets 1, 2 corresponds substantially to the laser beam welding device sketched in FIG. 1, such that in terms of the configuration of this device, reference is made to the preceding description.

[0068] A further exemplary embodiment of a device for carrying out the laser beam welding method according to the invention is sketched in FIG. 3. The laser beam welding device comprises a laser beam generator 17, whose laser beam 6 is guided by means of a deflection mirror 18 or the like to a focussing lens 7. The focussed laser beam 6 is then guided by means of at least one oscillating deflection device to the joint, delimiting a smaller gap 3, of the steel sheets 1, 2 to be welded together in the butt joint. The oscillating deflection device can be formed in this case by one or a plurality of deflection mirrors 15, 15. The deflection mirror 15, 15 is provided with an oscillation actuator 16, 16 for example a piezo drive.

[0069] An additional material having austenite-stabilising properties in the form of a wire 11 is supplied to the melt bath 9 generated exclusively by means of the oscillating laser beam 6 via a wire feeding device 10, wherein the tip of the additional wire melts in the melt bath 9 or in the working point of the laser beam 6. By means of a gas supply line 8, whose outlet opening is directed to the melt bath 9, inert gas, e.g. argon and/or helium is applied to this melt bath.

[0070] Furthermore, the laser beam welding device according to FIG. 3 has a device by means of which the geometry of the weld seam 13 is detected and the oscillation frequency and/or the amplitude of the oscillating laser beam 6 are automatically varied as a function of the detected geometry of the weld seam 13. The geometry of the laser weld seam is for example detected by means of a sensor device 19 which has a camera and a laser line illumination, wherein the geometry of the weld seam 13, in particular different height profiles and their positions, are detected according to the triangulation method. Alternatively or additionally, the geometry of the weld seam 13 can also be detected by means of inductive measurement methods, in particular eddy current testing or an eddy current probe. The measurement signals of the sensor device are transferred to a computer 20, which evaluates the measurement signals and controls the oscillation actuator(s) 16, 16 as a function of the measurement signals of the sensor device.

[0071] The implementation of the invention is not limited to the exemplary embodiments sketched in the drawing. In fact, numerous variants are conceivable which make use of the invention also in the case of a design differing from the sketched examples, as is indicated in the enclosed claims. It is in particular in the scope of the invention to combine together individual or a plurality of the features of the exemplary embodiments explained on the basis of FIGS. 1 to 3.

LIST OF REFERENCE NUMERALS

[0072] 1 Steel sheet (plate) [0073] 2 Steel sheet (plate) [0074] 2 Steel sheet (plate) [0075] 3 Gap [0076] 4 Metallic coating made of Al, e.g. AlSi [0077] 5 Laser beam welding head [0078] 6 Laser beam [0079] 7 Focussing lens [0080] 8 Supply line for inert gas [0081] 9 Melt bath [0082] 10 Wire feeding device [0083] 11 Additional wire [0084] 12 Heating element [0085] 13 Weld seam [0086] 14 Arrows [0087] 15, 15 Diversion mirror (deflection mirror) [0088] 16, 16 Actuator [0089] 17 Laser beam generator [0090] 18 Deflection mirror [0091] 19 Sensor device [0092] 20 Computer (controller) [0093] d Thickness jump