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

Abstract

A method for laser welding one or more workpieces made of press-hardenable steel in a butt joint, wherein the workpiece or workpieces have a thickness of between 0.5 and 1.8 mm, and/or a jump in thickness of between 0.2 and 0.4 mm occurs at the butt joint, and wherein laser welding is carried out with the feed of filler wire into the molten bath, which is generated only by at least one laser beam. The filler wire contains at least one alloy element from the group comprising manganese, chromium, molybdenum, silicon and/or nickel, which alloy element promotes the formation of austenite in the molten bath generated by the laser beam, wherein said at least one alloy element is present in the filler wire in a mass percentage greater by at least 0.1 wt. % than in the steel of the workpiece or workpieces and the workpiece or workpieces used are non-coated or the coating thereof is partially removed.

Claims

1. A method for laser welding one or more workpieces made of press-hardenable steel in a butt joint, wherein the one or more workpieces have a thickness of between 0.5 and 1.8 mm, or a jump in thickness of between 0.2 and 0.4 mm occurs at the butt joint, or the one or more workpieces have a thickness of between 0.5 and 1.8 mm and a jump in thickness of between 0.2 and 0.4 mm occurs at the butt joint, comprising the steps of: generating a molten bath with only at least one laser beam; carrying out the laser welding with a feed of filler wire into the molten bath, wherein the filler wire contains at least one alloy element from the group comprising manganese, chromium, molybdenum, silicon and nickel, wherein the at least one alloy element promotes formation of austenite in the molten bath generated by the at least one laser beam, wherein the at least one alloy element is present in the filler wire in a mass percentage greater by at least 0.1 wt. % than in the press-hardenable steel of the one or more workpieces, wherein the one or more workpieces used are non-coated or partially de-coated by ablation of their coating before the laser welding in a peripheral region along an abutting to be welded together, wherein filler wire comprises: 0.05 to 0.15 wt. % C, 0.5 to 2.0 wt. % Si, 1.0 to 2.5 wt. % Mn, 0.5 to 2.0 wt. % Cr+Mo, and 1.0 to 4.0 wt. % Ni, remainder Fe and unavoidable impurities, and wherein the filler wire has a carbon mass percentage which is lower by at least 0.1 wt. % than the press-hardenable steel of the workpiece or workpieces.

2. The method according to claim 1, wherein the steel of the one or more workpieces has the following composition: 0.16 to 0.50 wt. % C, max. 0.40 wt. % Si, 0.50 to 2.00 wt. % Mn, max. 0.025 wt. % P, max. 0.010 wt. % S, max. 0.60 wt. % Cr, max. 0.50 wt. % Mo, max. 0.050 wt. % Ti, 0.0008 to 0.0070 wt. % B, and min. 0.010 wt. % Al, remainder Fe and unavoidable impurities.

3. The method according to claim 1, wherein the filler wire is fed in a heated state to the molten bath.

4. The method according to claim 3, wherein at least a length portion of the filler wire is heated to a temperature of at least 50? C. before it is fed into the molten bath.

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

6. The method according to claim 5, wherein the inert gas used is pure argon or a mixture of argon and carbon dioxide.

7. The method according to claim 1, wherein the partially de-coated one or more workpieces used have a surface layer based on aluminium or on aluminium-silicon.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a perspective view of parts of a device for implementing the laser welding method according to the invention, two press-hardenable steel blanks of substantially the same thickness being welded together in a butt joint; and

(2) FIG. 2 is a perspective view of parts of a device for implementing the laser welding method according to the invention, in which two press-hardenable steel blanks which differ in thickness being welded together in a butt joint.

DETAILED DESCRIPTION OF THE INVENTION

(3) FIG. 1 schematically shows a device using which a laser welding method according to the invention can be carried out. The device comprises a support (not shown), on which two steel strips or steel blanks 1, 2 of different material qualities butt one another along the joint 3. For example, one workpiece 1 or 2 has a relatively soft drawing grade, while 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 from manganese-boron steel.

(4) The workpieces 1, 2 are substantially of the same thickness. The thickness thereof is between 0.5 and 1.8 mm, for example 1.6 mm.

(5) Outlined above the workpieces 1, 2 is a portion of a laser welding head 4 which is provided with both an optical system (not shown) to supply a laser beam, and a focussing lens 5 for the laser beam 6. Furthermore, a line 7 is arranged on the laser welding head 4 to supply inert gas. The opening of the inert gas line 7 is substantially directed onto the focusing region of the laser beam 6 or onto the molten bath 8 generated by the laser beam 6. The inert gas used is preferably pure argon or, for example, a mixture of argon, helium and/or carbon dioxide. In addition, associated with the laser welding head 4 is a wire feed apparatus 9, by which a specific additional material in the form of a wire 10 is fed into the molten bath 8 and is also melted by the laser beam 6. The filler wire 10 is fed in a heated state into the molten bath 8. For this purpose, the wire feed apparatus 9 is fitted with at least one heating element 11, for example with a heating coil which surrounds the wire 10. The heating element 11 preferably heats the filler wire 10 to a temperature of at least 50? C., more preferably to at least 90? C.

(6) The embodiment shown in FIG. 2 differs from the embodiment according to FIG. 1 in that the workpieces 1, 2 are of different thicknesses, so there is a jump in thickness d of between 0.2 and 0.4 mm, for example 0.3 mm at the butt joint 3. For example, one workpiece 2 has a sheet thickness within a range of 1.2 mm to 1.3 mm, while the other workpiece 1 has a sheet thickness within a range of 1.4 mm to 1.5 mm. Furthermore, the workpieces 1, 2 which are to be joined together in the butt joint 3 can also differ from one another in their material quality. For example, the thicker blank 1 is produced from higher-strength sheet steel, whereas the thinner steel blank 2 has a relatively soft drawing grade.

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

(8) The workpieces or steel blanks 1, 2 and 2 can be uncoated or can be provided with a coating, in particular with an AlSi layer. In the delivery state, i.e. before a heat treatment and rapid cooling, the yield strength 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 hot forming (press hardening), i.e. after austenitising at approximately 900 to 920? C. and subsequent rapid cooling, these steel blanks have a yield strength Re of approximately 1,100 MPa, a tensile strength Rm of approximately 1,500 to 2000 MPa and an elongation at break A.sub.80 of approximately 5.0%.

(9) If the workpieces or steel blanks 1, 2 and/or 2 are provided with an aluminium coating, in particular with an AlSi coating, said coating is ablated or partly removed before laser welding in the peripheral region along the abutting ends to be welded together. Aluminium coating material which adheres to the abutting ends or cut edges 3 is also removed, if appropriate. The aluminium coating material can preferably be ablated (removed) by means of at least one laser beam.

(10) The filler wire 10 used has the following chemical composition, for example: 0.1 wt. % C, 0.8 wt. % Si. 1.8 wt. % Mn, 0.35 wt. % Cr, 0.6 wt. % Mo, and 2.25 wt. % Ni, remainder Fe and unavoidable impurities.

(11) The manganese content of the filler wire 10 is always greater than the manganese content of the press-hardenable workpieces 1, 2 and/or 2. The manganese content of the filler wire 10 is preferably approximately 0.2 wt. % higher than the manganese content of the press-hardenable workpieces 1, 2 and/or 2. Furthermore, it is favourable if the content of chromium and molybdenum of the filler wire 10 is also higher than in the press-hardenable workpieces 1, 2 and/or 2. The combined chromium-molybdenum content of the filler wire 10 is preferably approximately 0.2 wt. % higher than the combined chromium-molybdenum content of the press-hardenable workpieces 1, 2 and/or 2. The nickel content of the filler wire 10 is preferably within a range of 1 to 4 wt. %. Furthermore, the filler wire 10 preferably has a lower carbon content than the press-hardenable steel of the workpieces 1, 2 and/or 2.