Method and Device for Fusion Welding One or a Plurality of Steel Sheets Made of Press-Hardenable Steel

Abstract

A method and a device for fusion welding one or more steel sheets made of press-hardenable steel, preferably manganese-boron steel; are disclosed. In the method, the fusion welding is performed by supplying filler wire into a molten bath generated a laser beam. In order to improve the hardenability of the weld seam, regardless of whether the steel sheets to be welded to one another are steel sheets of identical or different material quality, the filler wire is coated with graphite particles prior to fusion welding and the filler wire coated in this manner is introduced directly into the molten bath in such a way that the tip of the filler wire melts in the molten bath, the graphite particles are mixed with a waxy or liquid carrier medium to be applied to the filler wire, and the mixture is applied in the form of a coating to the filler wire. The method and the corresponding device are distinguished by a high productivity and a relatively low energy consumption. The method can be implemented with a relatively low equipment outlay.

Claims

1. A method of fusion welding one or a plurality of steel sheets made of press-hardenable steel, comprising ; supplying filler wire into a molten bath generated by a laser beam, wherein the filler wire is coated with graphite particles prior to fusion welding and the coated filler wire is introduced directly into the molten bath such that a tip of the filler wire melts in the molten bath, and wherein the graphite particles are mixed with a waxy or liquid carrier medium to be applied on the filler wire and the mixture is applied as a coating on the filler wire.

2. The method according to claim 1, wherein the filler wire is coated with the graphite particles at the location of the fusion welding.

3. The method according to claim 1, wherein the filler wire is coated with the graphite particles between a wire feed device and a guide line supplying the filler wire to the molten bath.

4. The method according to claim 1, wherein the filler wire is coated with the graphite particles by a coating device in the form of one of a dipping bath, a roller application device, and a spraying device.

5. The method according to claim 1, wherein oil is used as the liquid carrier medium.

6. The method according to claim 1, wherein the steel sheets have an aluminium or aluminium-silicone-based surface coating which extends to at least one longitudinal edge of the steel sheets.

7. The method according to claim 1, wherein the steel sheets have a thickness of at least 1.8 mm or at least 2.0 mm.

8. The method according to claim 1, wherein the steel sheets are welded a butt joint, and wherein a thickness of at least 0.4 mm results at the butt joint.

9. The method according to claim 1, wherein a proportion of the graphite particles in the mixture of the liquid carrier medium and the graphite particles is set such that the filler wire, after the mixture has been applied on the filler wire as the coating, has a carbon mass proportion of at least 0.2% by weight.

10. The method according to claim 1, wherein a proportion of the graphite particles in the mixture of the liquid carrier medium and the graphite particles is set such that the filler wire, after the mixture has been applied on the filler wire as the coating, has a carbon mass proportion which is higher by 0.1% by weight to 1.2% by weight than the carbon mass proportion of a base material of the steel sheets.

11. The method according to claim 1, wherein the filler wire when uncoated, contains at least one alloy element which favours the formation of austenite in the molten bath generated with the laser beam.

12. The method according to claim 1, wherein inert gas is applied to the molten bath during the fusion welding.

13. A device for fusion welding one or a plurality of steel sheets comprising: a laser welding head;, a guide line to supply filler wire into a molten bath generated by a laser beam; and by a coating device in the form of a dipping bath by which the filler wire is coated with a waxy or liquid mixture containing graphite particles.

14. The device for fusion welding according to claim 13, wherein the coating device is arranged between a wire feed device and a guide line supplying the filler wire to the molten bath.

15. The device for fusion welding according to claim 13, wherein the coating device is configured in the form of one of the dipping bath, a roller application device, and a spraying device.

16. The method according to claim 1, wherein the press-hardenable steel is manganese boron steel.

17. The method according to claim 2, wherein the filler wire is coated continuously with the graphite particles at the location of the fusion welding.

18. The method according to claim 5, wherein the oil is paraffin oil.

Description

[0040] The invention is explained in detail below on the basis of a drawing representing a plurality of exemplary embodiments. They show schematically:

[0041] FIG. 1 a perspective representation of parts of a device for carrying out the fusion welding method according to the invention, with two substantially equally thick, press-hardenable steel sheets being welded together in the butt joint by means of a laser beam using filler wire;

[0042] FIG. 2 a perspective view of parts of a device for carrying out the fusion welding method according to the invention, with two differently thick, press-hardenable steel sheets being welded together in the butt joint by means of a laser beam using filler wire;

[0043] FIG. 3 a longitudinal sectional view of a coating device for coating a filler wire for a laser welding device according to FIG. 1 or FIG. 2;

[0044] FIG. 4 a further exemplary embodiment of a coating device for coating a filler wire for a laser welding device according to FIG. 1 or FIG. 2, in a front or side view, with a wire supplying line being represented in the longitudinal section; and

[0045] FIG. 5 another exemplary embodiment of a coating device for coating a filler wire for a laser welding device according to FIG. 1 or FIG. 2, in a front or side view, with the components of the device being represented partially in a vertical section.

[0046] A laser welding device is sketched in FIGS. 1 and 2, 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 steel, preferably 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. 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.

[0047] The press-hardenable steel, of which at least one of the steel sheets 1, 2 to be connected to one another consists, can for example have the following chemical composition: [0048] Max. 0.45% by weight C, [0049] Max 0.40% by weight Si, [0050] Max 2.0% by weight Mn, [0051] Max 0.025% by weight P, [0052] Max 0.010% by weight S, [0053] Max 0.8% by weight Cr+Mo, [0054] Max 0.05% by weight Ti, [0055] Max 0.0050% by weight B, and [0056] Min 0.010% by weight Al, [0057] Remainder Fe and unavoidable impurities.

[0058] 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 Aso of approx. 5.0%.

[0059] The steel sheets 1, 2 are preferably provided with a metallic coating 4 made of aluminium or zinc. It is for example an AlSi coating. The metallic surface coating 4 is applied to the base material preferably on both sides, for example by hot dip coating, by guiding a strip made of press-hardenable steel, preferably manganese-boron steel through a zinc or AlSi molten bath, blowing off excessive coating material from the strip and the coated strip then subsequently treated, in particular heated. The aluminium content of the surface coating 4 can be in the range of 70 to 90% by weight.

[0060] Alternatively, only one of the steel sheets 1, 2 to be welded can also have an aluminium or zinc-containing surface coating 4. Furthermore, the surface 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.

[0061] The steel sheets 1, 2 can, as shown in FIG. 1, have substantially 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 m, in particular less than 50 m.

[0062] A section of a laser 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 in the range of 5 to 6 kW.

[0063] A line 8 for supplying inert gas is assigned to the laser welding head 5. The discharge of the inert gas line 8 is substantially directed to the molten bath 9 generated with the laser beam 6 and the weld seam 14. A pressurised gas tank serving as the inert gas source is designated with 8.1. Pure argon or for example a mixture of argon, helium and/or carbon dioxide is preferably used as the inert gas.

[0064] In addition, a guide line 10 is assigned to the laser welding head 5 by means of which a filler material in the form of a wire 11 is supplied to the molten bath 9, with the tip of the filler wire 11 being melted in the molten bath 9. The filler wire 11 contains substantially no aluminium. It may for example have the following chemical composition: [0065] 0.1% by weight C, [0066] 0.8% by weight Si, [0067] 1.8% by weight Mn, [0068] 0.35% by weight Cr, [0069] 0.6% by weight Mo, and [0070] 2.25% by weight Ni, [0071] Remainder Fe and unavoidable impurities.

[0072] The exemplary 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 of 1.6 mm to 3.0 mm. Moreover, the steel sheets 1, 2 to be connected together 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 to one another with a gap 3 of a few tenths of a millimetre.

[0073] According to the invention, the laser welding device comprises a coating device 12 by means of which the filler wire 11 is coated with a waxy or liquid mixture containing graphite particles. The coating device 12 indicated in FIGS. 1 and 2 only in the form of a box can be implemented in different embodiments. It is preferably arranged between a wire feed device 13 and the guide line 10 supplying the filler wire 11 to the molten bath 9 (cf. FIGS. 3 to 5).

[0074] An exemplary embodiment is represented in FIG. 3, in which the coating device 12 has a chamber 12.1 as a reservoir for receiving a liquid coating agent. The chamber 12.1 therefore contains a dipping bath 15 formed of liquid coating agent. The coating agent is supplied to the chamber 12.1 via an inlet opening 12.2, which discharges for example into the chamber 12.1 close to its bottom 12.3. The liquid coating agent is a mixture of a liquid carrier medium and graphite particles. The carrier medium is preferably oil, particularly preferably paraffin oil, for example so-called white oil.

[0075] The chamber 12.1 has an inlet 12.4 and an outlet 12.5 to channel a filler wire 11 to be coated. A wire feed device 13 is arranged upstream of the inlet 12.4 which has at least one drive roller 13.1 and a counter roller 13.2 which abut on the filler wire 11 with a certain pressing force.

[0076] The opening or cross-sectional surface of the outlet 12.5 is greater by a certain extent than the cross-sectional surface of the uncoated filler wire 11. The outlet 12.5 and the filler wire 11 therefore delimit an annular gap 12.6, whose radial gap dimension corresponds roughly to the desired thickness of a shell-shaped coating 11.1 to be applied on the filler wire 11. The gap dimension of the annular gap 12.6 is selected corresponding to the coating material quantity to be applied. Alternatively or additionally, the outlet 12.5 of the chamber 12.1 can be provided with a variably settable annular orifice by means of which the gap dimension of the annular gap 12.6 present between the filler wire 11 and the outlet opening 12.5 is variably, preferably continuously settable.

[0077] The inlet 12.4, through which the filler wire 11 to be coated enters the chamber 12.1, is arranged and dimensioned such that the filler wire 11 is guided as concentrically as possible to the inner wall of the outlet 12.5. The inlet 12.4 can to this end be delimited by a slide guide 12.41.

[0078] The outlet 12.5 of the chamber 12.1 can for example be defined by a sleeve 12.7 which preferably has a cylindrical inner wall and protrudes into the interior of the chamber 12.1. Alternatively, the sleeve 12.7 could also protrude at the outside, e.g. at the underside of the chamber 12.1. The guide line 10 guiding the coated filler wire 11 during the fusion welding process to the molten bath adjoins to the outlet 12.5. Moreover, the chamber 12.1 is provided above the dipping bath level 15.1 preferably with at least one venting or pressure compensation opening 12.8.

[0079] A further exemplary embodiment of a coating device 12 is represented in FIG. 4, by means of which filler wire 11 to be supplied to the molten bath 9 is coated during the fusion welding process. In this example, the coating device 12 is configured as a roller application system. The coating device 12 has at least one trough-shaped container 12.9 to receive liquid coating agent 15. The coating agent 15 is in turn a mixture of a liquid carrier medium and graphite particles. The carrier medium can in particular be paraffin oil, for example white oil.

[0080] At least one take-up roller 12.10 dipped partially into the coating agent 15 is assigned to the trough-shaped container 12.9 which transfers coating agent 15 received from the container 12.9 onto an application roller 12.11.

[0081] The application roller 12.11 is preferably provided with an annular groove (not shown), whose cross-sectional profile is greater by a certain extent than the cross-sectional profile of the filler wire 11 to be coated, with the filler wire 11 being guided such that it engages at least partially into the annular groove of the application roller 12.11. The take-up roller 12.10 can in this case have a circumferential projection (not shown) which also engages into the annular groove.

[0082] The quantity of the coating material to be applied can be or is set by controlling the rotational speed of the at least one application roller 12.11 and/or the at least one take-up roller 12.10 dipped into the coating agent in relation to the feed speed of the filler wire 11.

[0083] The filler wire 11 coated by means of the at least one application roller 12.11 or a plurality of such application rollers 12.11 is then supplied to the molten bath 9 generated by means of the laser beam 6 by the guide line 10. The wire feed device 13 arranged upstream of the coating device 12 according to FIG. 4 in turn has at least one drive roller 13.1 and a counter roller 13.2 which abut on the filler wire 11 with a frictional connection.

[0084] A further exemplary embodiment of a coating device 12 is represented in FIG. 5 by means of which filler wire 11 to be supplied to the molten bath 9 is coated during the fusion welding process. The coating device 12 in this case comprises a dipping bath 15 in which at least one deflection roller 12.12 is arranged immersed, such that the filler wire 11 conveyed in the direction of the guide line 10 by means of a wire feed device 13 arranged upstream of the dipping bath 15 is guided channelled the dipping bath 15. Further optional deflection rollers are designated with 12.13 which are arranged outside of the dipping bath 15. The deflection rollers 12.12, 12.13 are preferably provided with an annular groove (not shown), whose cross-sectional profile (width) is greater by a certain extent than the cross-sectional profile (diameter) of the filler wire 11 to be coated.

[0085] A stripping or layer thickness setting device 16 can be arranged between the deflection roller 12.2, arranged in the dipping bath 15, and the guide line 10, by means of which device the thickness or quantity of the coating material to be applied can be set. The stripping or layer thickness setting device 16 can for example be formed of at least one annular stripping screen and/or an inert gas or pressurised air nozzle (not shown) directed on the coated filler wire 11. The gap dimension of the annular gap between stripping screen and filler wire 11 is selected according to the coating material quantity to be applied. The stripping screen 16 is preferably variably settable, thus the gap dimension of the annular gap 12.6 present between the filler wire and the stripping screen is variably, preferably continuously settable. Excess coating material (coating agent) falls from the filler wire 11 on the stripping or layer thickness setting device 16 back into the dipping bath container 15.2.

[0086] Alternatively, the inlet opening of the guide line 10 can also be arranged in the dipping bath 15 such that the end of the guide line 10 immersed in the dipping bath 15 assumes the function of a layer thickness setting device.

[0087] The execution 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 in the case of a configuration 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 5.

LIST OF REFERENCE NUMERALS

[0088] 1 steel sheet (workpiece) [0089] 2 steel sheet (workpiece) [0090] 2 steel sheet (workpiece) [0091] 3 gap (joint gap) [0092] 4 metallic coating, e.g. made of Al, AlSi or Zn [0093] 5 laser welding head [0094] 6 laser beam [0095] 7 focussing lens [0096] 8 supply line for inert gas [0097] 8.1 inert gas supply [0098] 9 molten bath [0099] 10 guide line (filler wire supplying device) [0100] 11 filler wire [0101] 11.1 coating of 11 [0102] 12 coating device [0103] 12.1 chamber [0104] 12.2 inlet opening [0105] 12.3 base [0106] 12.4 inlet [0107] 12.41 slide guide [0108] 12.5 outlet [0109] 12.6 annular gap [0110] 12.7 sleeve [0111] 12.8 venting or pressure compensation opening [0112] 12.9 trough-shaped container [0113] 12.10 take-up roller [0114] 12.11 application roller [0115] 12.12 deflection roller [0116] 12.13 deflection roller [0117] 13 wire feed device [0118] 13.1 drive roller [0119] 13.2 counter roller [0120] 14 weld seam [0121] 15 dipping bath (coating agent) [0122] 15 coating agent [0123] 15.1 dipping bath level [0124] 15.2 dipping bath vessel (trough-shaped container) [0125] 16 stripping or layer thickness setting device [0126] d thickness jump