Method for Fusion Welding of One or More Steel Sheets of Press-Hardenable Steel

Abstract

A method for fusion welding of one or more steel sheets (1, 2) made of press-hardened steel, preferably manganese-boron steel is disclosed. At least one of the steel sheets has a metallic coating (4) which contains aluminum, and the fusion welding is performed while filler material (11) is being fed into the molten bath (9). In order to improve the hardenability of the weld seam (14), irrespective of whether the steel sheets to be welded together are steel sheets of the same or different material grades and/or steel sheets of different sheet thicknesses, a single laser focal spot (16) with different energy distribution is generated on the molten bath by means of one or more optical elements such that the laser focal spot (16) has a smaller laser focal spot area (16.1) and a larger laser focal spot area (16.2).

Claims

1. A method for fusion welding of one or more steel sheets made of press-hardened steel, wherein at least one of the steel sheets has a metallic coating which contains aluminum, and wherein the fusion welding is performed while filler material is being fed into the molten bath produced by at least one laser beam, the method comprising: generating a single laser focal spot with different energy distribution by of one or a plurality of optical elements on the molten bath such that the laser focal spot has a smaller laser focal spot area and a larger laser focal spot area, irradiating a first surface with the larger laser focal spot area wherein the first surface is at least two times of a second surface irradiated by the smaller laser focal spot area, and introducing a higher laser energy output per surface unit in the smaller laser focal spot area than in the larger laser focal spot area.

2. A method according to claim 1, wherein the laser beam is substantially free of oscillation during fusion welding.

3. A method according to claim 1, wherein the optical element, by which the laser focal spot having the different energy distribution is produced, is configured such that the position of the smaller laser focal spot area within the larger laser focal spot area is adjustable relative to the larger laser focal spot.

4. A method according to claim 3, wherein the position of the smaller laser focal spot area within the larger laser focal spot area is adjusted in a direction running one of parallel and transverse to a welding direction.

5. A method according to claim 1, wherein the larger laser focal spot area has an elongated shape, and wherein a longitudinal axis of the larger focal spot area runs substantially in a welding direction.

6. A method according to claim 1, wherein the larger laser focal spot area has a longitudinal extension that is at least 2 times, the average diameter or largest diameter of the smaller laser focal spot area.

7. A method according to claim 1, wherein the filler material is supplied in the form of a wire or powder.

8. A method according to claim 1, wherein the filler material does not contain any aluminum except for unavoidable impurities or unavoidable trace amounts.

9. A method according to claim 1, wherein the filler material contains at least one alloy element of a group comprising nickel, chromium, and carbon.

10. A method according to claim 1, wherein the filler material has the following composition: 0.05-0.4% by weight C, 0-2.0% by weight Si, 0-3.0% by weight Mn, 4-25% by weight Cr, 0-0.5% by weight Mo, and 5-12% by weight Ni, the remainder consisting of Fe and unavoidable impurities.

11. A method according to claim 1, wherein the press hardened steel has the following composition: 0.10-0.50% by weight C, max. 0.40% by weight Si, 0.50-2.0% by weight Mn, max. 0.025% by weight P, max. 0.010% by weight S, max. 0.60% by weight Cr, max. 0.50% by weight Mo, max. 0.050% by weight Ti, 0.0008-0.0070% by weight B, and min. 0.010% by weight Al, the remainder consisting of Fe and unavoidable impurities.

12. A method according to claim 1, wherein the steel sheets are joined in a butt joint, wherein a gap with an average gap width in the range from 0.01 to 0.15 mm is set on the butt joint to be joined.

13. A method according to claim 1, wherein the steel sheets are joined with a welding speed of at least 4 m/min.

14. A method according to claim 1, wherein the filler material is supplied in the form of a wire, and wherein the wire is fed at a supply speed in the range of 40% to 90% of a welding speed.

15. A method according to claim 1, wherein the molten bath is not exposed to a protective gas flow during laser welding at least on a side facing the laser beam.

16. A method according to claim 1, wherein the filler material is fed into the molten bath such that the filler material is fed directly into the smaller laser focal spot area.

17. A method according to claim 1, wherein the filler material is supplied in a dragging manner.

18. A method according to claim 1, wherein the filler material supplied in the form of a wire is supplied to the molten bath in such that a central axis of the wire with a surface of the at least one steel sheet to be welded or of the steel sheets to be welded together encloses an acute angle of less than 50°.

19. A method according to claim 1, wherein the filler material is heated to a temperature of at least 60° C., by a heating device before being fed into the molten bath.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0049] The invention is explained in more detail in the following with reference to a drawing illustrating exemplary embodiments. The drawings show schematically the following:

[0050] FIG. 1 shows a perspective view of parts of a device for carrying out the fusion welding method according to the invention, wherein two press-hardenable steel sheets of essentially the same thickness are welded together in a butt joint using filler wire by means of a laser beam;

[0051] FIG. 2 shows a perspective view of parts of a device for carrying out the fusion welding method according to the invention, wherein two press-hardenable steel sheets of essentially different thicknesses are welded together in a butt joint using filler wire by means of a laser beam;

[0052] FIG. 3 shows a perspective view of parts of a further device for carrying out the fusion welding method according to the invention, wherein two press-hardenable steel sheets of essentially the same thickness are welded together in a butt joint using filler wire by means of a laser beam;

[0053] FIG. 4a shows a planar view of a laser focal spot generated when carrying out the method according to the invention; and

[0054] FIG. 4b shows a planar view of a further laser focal spot generated when carrying out the method according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0055] Laser beam welding devices for carrying out the method according to the invention are sketched in FIGS. 1 to 3. The respective device comprises a base (not shown), on which two strips or plates 1, 2 made of steel of the same or different material grade are arranged such that their edges to be welded together are arranged in a butt joint. At least one of the steel sheets 1, 2 is made of press-hardenable steel, preferably manganese-boron steel. The steel sheets 1, 2 are joined with the smallest possible gap 3 of a few tenths of a millimeter in the butt joint. For example, the width of the gap 3 is less than 0.2 mm, preferably less than 0.15 mm. If the steel sheets 1, 2 are manufactured from steel of different material grades, one steel sheet 1 or 2 has, for example, a relatively soft deep-drawing grade, while the other steel sheet 2 or 1 consists of higher-strength steel.

[0056] The press-hardenable steel, of which of at least one of the steel sheets 1, 2 to be connected to one another consists, can for example have the following chemical composition: [0057] 0.10 to 0.50% by weight C, [0058] max. 0.40% by weight Si, [0059] 0.50 to 2.0% by weight Mn, [0060] max. 0.025% by weight P, [0061] max. 0.010% by weight S, [0062] max. 0.60% by weight Cr, [0063] max. 0.50% by weight Mo, [0064] max. 0.050% by weight Ti, [0065] 0.0008 to 0.0070% by weight B, and [0066] min. 0.010% by weight Al, [0067] the remainder consisting of Fe and unavoidable impurities.

[0068] In delivery condition, i.e. before heat treatment and rapid cooling, the press-hardenable steel sheet 1 or 2 has a yield strength Re of preferably at least 300 MPa; its tensile strength Rm is, for example, at least 480 MPa, and its elongation at fracture A.sub.80 is preferably at least 10%. After hot forming (press hardening), i.e. heating to austenitising temperature of approx. 900° C. to 950° C., forming at this temperature and then rapid cooling, the press-hardened sheet steel has a yield strength Re of approx. 1,100 MPa, a tensile strength Rm of approx. 1,500 MPa to 2,000 MPa and an elongation at fracture A.sub.80 of approx. 5%.

[0069] The steel sheets 1, 2 are provided with a metallic coating 4 made of aluminum. This is, for example, an Al—Si coating. The coating 4 is preferably applied to the base material on both sides, for example by hot-dip coating, in which a strip of press-hardenable steel, preferably manganese-boron steel, is guided through an Al—Si molten bath, excess coating material is blown off the strip and the coated strip is subsequently treated, in particular heated. The aluminum content of the coating 4 can be in the range of 70% by weight to 90% by weight.

[0070] Alternatively, only one of the steel sheets 1, 2 to be welded can have an aluminum-containing coating 4. Furthermore, the coating 4 can, if necessary, only be applied to 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.

[0071] The steel sheets 1, 2 can, as shown in FIGS. 1 and 3, be essentially the same thickness. The sheet thickness is, for example, in the range of 0.8 to 3.0 mm, preferably in the range of 1.8 mm to 3.0 mm, while the thickness of the metallic surface coating 4 on the respective sheet side can be less than 100 μm, in particular less than 50 μm.

[0072] Above the steel sheets 1, 2, a section of a laser welding head 5 is shown, which is provided with optics for the shaping and alignment of 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 provides power in the range of 5 kW to 10 kW.

[0073] A line 8 for the supply of shielding gas can optionally be assigned to the laser welding head 5. The mouth of the protective gas line 8 is or is for example essentially directed at the freshly generated section of the weld seam 14 in such a way that the molten bath 9 itself is not, or at least is not directly, exposed to the protective gas flow. 8.1 is a compressed gas tank serving as a protective gas source. Pure argon or, for example, a mixture of argon, helium and/or carbon dioxide is preferably used as a protective gas. An alternative or further configuration (not shown) of the fusion welding method envisages the underside or the side of the molten bath 9 facing away from the laser beam 6 and the underside of the weld seam 14 being exposed to protective gas.

[0074] Furthermore, a guide line 10 is assigned to the laser welding head 5 by means of which filler material (filler metal) 11 is supplied to the molten bath 9 for example in the form of a wire, wherein the tip of the wire 11 melts in the molten bath 9. The filler metal 11 essentially does not contain any aluminum. It has, for example, the following chemical composition: [0075] 0.05 to 0.4% by weight C, [0076] 0 to 2.0% by weight Si, [0077] 0 to 3.0% by weight Mn, [0078] 4 to 25% by weight Cr, [0079] 0 to 0.5% by weight Mo, and [0080] 5 to 12% by weight Ni, [0081] the remainder consisting of Fe and unavoidable impurities.

[0082] Instead of a wire-shaped filler metal (filler wire) 11, a powdered filler metal in the form of a gas powder flow can also be supplied to the molten bath 9. The powdered filler metal can have the same chemical composition as the filler wire 11 described above. One of the above-mentioned protective gases is preferably used as carrier gas for feeding the powdered filler metal into the molten bath 9.

[0083] According to the invention, the laser welding head 5 has one or a plurality of optical elements by means of which a single laser focal spot 16 with different energy distribution on the molten bath 9 is generated such that the laser focal spot 16 has a smaller laser focal spot area 16.1 and a larger laser focal spot area 16.2 (see also FIGS. 4a and 4b). The larger laser focal spot area 16.2 irradiates a surface which is at least 2 times, preferably at least 3 times, of a surface irradiated by the smaller laser focal spot area 16.1, wherein a higher laser energy output per surface unit is introduced in the smaller laser focal spot area 16.1 than in the larger laser focal spot area 16.2. The smaller laser focal spot area 16.1 and the larger laser focal spot area 16.2 can have different energy levels, which are independent of one another. For example, both the smaller laser focal spot area 16.1 and the larger laser focal spot area 16.2 can have a laser energy output in the range of 4 kW to 5 kW, whereby this energy or output is distributed over a significantly larger area in the larger laser focal spot area 16.2. The smaller laser focal spot area 16.1 is essentially used for deep welding, while the larger laser focal spot area 16.2 supports the welding process.

[0084] The larger laser focal spot area 16.2 has an elongated shape, for example an oval, elliptical or rectangular shape. Its longitudinal axis runs essentially in the respective welding direction WD, i.e. essentially parallel thereto. The smaller laser focal spot area 16.1 can have an essentially circular shape or also an elongated shape (see FIGS. 4a and 4b).

[0085] The optical element(s) of the laser welding head 5, by means of which the laser focal spot having a different energy distribution is generated, can for example be a diffractory or refractory optical element assigned to the focussing lens 7 and/or a smaller additional focussing lens 7.1 (see FIGS. 1 and 2).

[0086] Another possibility for generating a single laser focal spot 16 with different energy distribution is shown in FIG. 3. The laser welding head 5 shown there schematically has a focussing lens 7 with a light guide or light fibre bundle 7.2 associated thereto.

[0087] Preferably, the optical elements 7, 7.1 or 7, 7.2 of the laser welding head 5 are designed in such a way that the position of the smaller laser focal spot area 16.1 can be adjusted within the larger laser focal spot area 16.2 relative to the latter. For example, the position of the smaller laser focal spot area 16.1 within the larger laser focal spot area 16.2 can be adjusted in a direction running parallel and/or transverse to the welding direction WD (X direction and/or Y direction). This adjustment option is schematically indicated in FIGS. 4a and 4b by dashed double arrows 18, 19. For example, the smaller focussing lens 7.1, the at least one diffractory or refractory optical element or the light guide 7.2 is mounted in the laser welding head 5 radially adjustable to the focussing lens 7.

[0088] If the different energy distribution in the laser focal spot is achieved by means of a focussing lens 7 and a light guide or light fibre bundle 7.2 assigned to the focussing lens, the position of the laser focal spot areas 16.1 and 16.2 relative to one another can for example be varied by defocussing the laser beam 6.

[0089] Furthermore, it can be seen in FIG. 4a as well as in FIG. 4b that the larger laser focal spot area 16.2 has a longitudinal extension which is at least 2 times, preferably at least 2.5 times, particularly preferably at least 3 times the average diameter or the largest diameter of the smaller laser focal spot area 16.1.

[0090] The exemplary embodiment shown in FIG. 2 differs from the examples shown in FIGS. 1 and 3 in that the steel sheets 1, 2′ are of different thicknesses such that a thickness step d is present on the butt joint. For example, one sheet 2′ has a sheet thickness in the range of 0.8 mm to 1.2 mm, while the other sheet 1 has a sheet thickness in the range of 1.6 mm to 3.0 mm. In addition, the steel sheets 1, 2′ to be joined together in the butt joint can also differ in their material quality. For example, the thicker plate 1 is made of higher-strength steel, whereas the thinner steel plate 2′ has a relatively soft deep-drawing quality. The steel sheets 1, 2′ are also joined together with the smallest possible gap 3 of a few tenths of a millimetres.

[0091] In FIG. 2, as illustrated in the embodiment, during laser welding, the molten bath 9 is not exposed to a protective gas flow on its side facing the laser beam 6. However, the side of the molten bath 9 facing away from the laser beam 6 and the side of the weld seam 14 facing away from the laser beam 6 are preferably supplied with protective gas.

[0092] The described special or adapted energy distribution in the individual laser focal spot 16 has the effect that the temperature distribution and thus the flows in the molten bath 9 change. This results in better homogenisation of the weld seam 14. Welding speeds of 5 m/min and more are thereby advantageous for the homogeneity of the weld seam 14. The filler wire 11 is preferably fed at a speed of 40% to 90% of the welding speed.

[0093] The filler wire 11 is fed into the molten bath 9 preferably in such a way that the wire 11 touches the smaller laser focal spot area 16.2 or is directed essentially directly at the smaller laser focal spot area 16.2. In addition, the wire feed is preferably in a dragging manner (see FIG. 1 and FIG. 3).

[0094] Furthermore, it can be seen in FIGS. 1 to 3 that the filler wire 11 is fed into the molten bath 9 in such a way that the central axis of the wire 11 with the surface of the at least one steel sheet 1, 2 to be welded or of the steel sheets 1, 2 to be welded together encloses an acute angle, which for example lies in a range of 10° to 45°, preferably in the range between 10° and 30°.

[0095] The implementation of the invention is not limited to the exemplary embodiments schematically represented in the drawing. Instead, numerous variants are conceivable that also make use of the invention specified in the attached claims in the case of a design deviating from the examples shown. For example, it is also possible in the context of the invention for the filler material 11, in particular in the form of a wire, to be heated to a temperature of at least 60° C. by means of a heating device before flowing into the molten bath 9. For example, the filler wire 11 is heated to a temperature in the range of 100° C. to 300° C., preferably in the range of 150° C. to 250° C. before flowing into the molten bath 9.