Method for laser welding one or more workpieces of hardenable steel in a butt joint

Abstract

A method for laser welding of one or more workpieces made from press hardenable steel, in a butt joint, in which the workpiece or the workpieces have a thickness of at least 1.8 mm and/or a jump in thickness of at least 0.4 mm arises at the butt joint including supplying filler wire into a molten bath generated by a laser beam. In order to ensure that the weld seam can reliably harden into a martensitic structure during the hot forming (press hardening), the filler wire contains at least one alloy element from the group of manganese, chromium, molybdenum, silicon and nickel, wherein the at least one alloy element is present in the filler wire with a mass proportion that is larger by .Iadd.at least .Iaddend.0.1% by weight than the mass proportion of the element in the press hardenable steel of the workpiece or the workpieces.

Claims

1. A method for laser welding of one or more workpieces made from press hardenable steel in a butt joint, in which the workpiece or the workpieces have a thickness of at least 1.8 mm and/or a jump in thickness of at least 0.4 mm arises at the butt joint, comprising supplying filler wire into a molten bath generated exclusively by a laser beam, wherein the filler wire contains at least one alloy element from the group consisting of manganese, chromium, molybdenum, silicon and nickel, the element promoting the formation of austenite in the molten bath generated using the laser beam, wherein the at least one alloy element is present in the filler wire with a mass proportion that is larger by .Iadd.at least .Iaddend.0.1% by weight than a mass proportion of the element in the press hardenable steel of the workpiece or the workpieces, wherein the filler wire has a carbon mass proportion that is lower by at least 0.1% by weight than the mass proportion of carbon in the press hardenable steel of the workpiece or the workpieces, and wherein the workpiece used or the workpieces used are uncoated or, by removing in the edge region along the abutting edges to be welded to one another before the laser welding, are partly de-layered.

2. The method according to claim 1, wherein the steel of the workpiece or the workpieces has the following composition: 0.10-0.50% by weight C, max. 0.40% by weight Si, 0.50-2.00% 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, max. 0.0008-0.0070% by weight B, and min. 0.010% by weight Al, remainder Fe and unavoidable impurities.

3. The method according to claim 1, wherein the filler wire has the following composition: 0.05-0.15% by weight C, 0.5-2.0% by weight Si, 1.0-2.5% by weight Mn, 0.5-2.0% by weight Cr+Mo, and 1.0-4.0% by weight Ni, remainder Fe and unavoidable impurities.

4. The method according to claim 1, .[.herein.]. .Iadd.wherein .Iaddend.the filler wire is supplied to the molten bath in a heated state.

5. The method according to claim 4, wherein the filler wire is heated to a temperature of at least 50 C., at least in a length section, before supply into the molten bath.

6. The method according to claim 1, wherein the molten bath is loaded with protective gas during the laser welding.

7. The method according to claim 6, wherein pure argon or a mixture of argon and carbon dioxide is used as the protective gas.

8. The method according to claim 1, wherein the partly de-layered workpiece or the partly de-layered workpieces have a surface layer comprising aluminium or aluminium/silicon.

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

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained in more detail below on the basis of a drawing illustrating exemplary embodiments. In the figures:

(2) FIG. 1 shows a perspective view of parts of a device for carrying out the laser welding method according to the invention, wherein two essentially equally thick, press hardenable steel blanks are welded to one another in a butt joint; and

(3) FIG. 2 shows a perspective view of parts of a device for carrying out the laser welding method according to the invention, wherein here two differently thick, press hardenable steel blanks are welded to one another in a butt joint.

DETAILED DESCRIPTION OF THE INVENTION

(4) A device is schematically illustrated in FIG. 1, using which a laser welding method according to the invention can be carried out. The device comprises a support (not shown), on which two strips or blanks 1, 2 made from steel of different material quality abut bluntly along the joint 3. For example, the one workpiece 1 or 2 has a relatively weak deep-drawing grade, whilst the other workpiece 2 or 1 consists of higher strength sheet steel. At least one of the workpieces 1, 2 is produced from press hardenable steel, for example made from manganese-boron steel.

(5) The workpieces 1, 2 are essentially of equal thickness. The thickness thereof is at least 1.8 mm, for example at least 2.0 mm.

(6) Sketched above the workpieces 1, 2 is a section of a laser welding head 4, which is provided with an optical system (not shown) for supplying a laser beam and also a focussing lens 5 for the laser beam 6. Furthermore, a pipe 7 for supplying protective gas is arranged on the laser welding head 4. The aperture of the protective gas pipe 7 is essentially directed onto the focus area of the laser beam 6 or the molten bath 8 generated with the laser beam 6. Pure argon or for example a mixture of argon, helium and/or carbon dioxide is preferably used as protective gas. In addition, a wire feed apparatus 9 is assigned to the laser welding head 4, by means of which, a special additional material is fed to the molten bath 8 in the form of a wire 10, which is likewise melted by the laser beam 6. The additional wire (filler wire) 10 is fed to the molten bath 8 in a heated state. To this end, the wire feed apparatus 9 is equipped with at least one heating element (not shown), for example a heating coil surrounding the wire 10. Using the heating element, the filler wire 10 is preferably heated to a temperature of at least 50 C., particularly preferably to at least 90 C.

(7) The exemplary embodiment illustrated in FIG. 2 differs from the exemplary embodiment according to FIG. 1 in that the workpieces 1, 2 are of different thickness, so that at the butt joint 3, a jump in thickness d of at least 0.4 mm is present. For example, the one workpiece 2 has a sheet thickness in the range from 0.5 mm to 1.2 mm, whilst the other workpiece 1 has a sheet thickness in the range from 1.6 mm to 2.5 mm. Furthermore, the workpieces 1, 2 to be connected to one another in the butt joint 3 can also differ from one another in terms of the material quality thereof. For example, the thicker blank 1 is produced from higher strength sheet steel, whereas the thinner steel blank 2 has a relatively weak deep-drawing quality.

(8) The press hardenable steel, from which at least one of the workpieces 1, 2 or 2 to be connected to one another in the butt joint 3, can for example have the following chemical composition: max. 0.45% by weight C, max. 0.40% by weight Si, max. 2.0% by weight Mn, max. 0.025% by weight P, max. 0.010% by weight S, max. 0.8% by weight Cr+Mo, max. 0.05% by weight Ti, max. 0.0050% by weight B, and min 0.010% by weight Al, remainder Fe and unavoidable impurities.

(9) The workpieces or steel blanks 1, 2 or 2 can be uncoated or provided with a coating, particularly an AlSi layer. In the delivery state, i.e. before a heat treatment and rapid cooling, the yield point Re of the press hardenable steel blanks 1, 2 and/or 2 is preferably at least 300 MPa; the tensile strength Rm thereof is at least 480 MPa, and the elongation at break A.sub.80 thereof is at least 10%. After the hot forming (press hardening), i.e. austenisation at approx. 900 to 920 C. and subsequent rapid cooling, these steel blanks have a yield point Re of approx. 1,100 MPa, a tensile strength Rm of approx. 1500 to 2000 MPa and an elongation at break A.sub.80 of approx. 5.0%.

(10) Insofar as the workpieces or steel blanks 1, 2 and/or 2 are provided with an aluminium coating, particularly with an AlSi coating, the coating can be removed or partly de-layered in the edge region along the abutting edges to be welded to one another, before the laser welding. If appropriate, aluminium coating material adhering at the abutting or intersection edges 3 is also removed. The removal (elimination) of the aluminium coating material can preferably take place by means of at least one laser beam.

(11) The filler wire 10 used .[.typically.]. has .Iadd.for example .Iaddend.the following chemical composition: 0.1% by weight C, 0.8% by weight Si, 1.8% by weight Mn, 0.35% by weight Cr, 0.6% by weight Mo, and 2.25% by weight Ni, remainder Fe and unavoidable impurities.

(12) The manganese content of the filler wire 10 is in this case constantly higher than the manganese content of the press hardenable workpieces 1, 2 or 2. Preferably, the manganese content of the filler wire 10 is in this case higher by approx. 0.2% by weight than the manganese content of the press hardenable workpieces 1, 2 or 2. Furthermore, it is beneficial, if also the chromium and molybdenum content of the filler wire 10 is higher than in the press hardenable workpieces 1, 2 or 2. Preferably, the combined chromium-molybdenum content of the filler wire 10 is in this case higher by approx. 0.2% by weight than the combined chromium-molybdenum content of the press hardenable workpieces 1, 2 or 2. The nickel content of the filler wire 10 preferably lies in the range of 1 to 4% by weight. Additionally, the filler wire 10 preferably has a lower carbon content than the press hardenable steel of the workpieces 1, 2 or 2.