EP2961560A1

EP2961560A1 - Method for joining steel plates or strips provided with a metal coating to one another by means of laser-beam welding

Info

Publication number: EP2961560A1
Application number: EP14704284.0A
Authority: EP
Inventors: Gerhard Alber; Kai Leibold
Original assignee: Wisco Lasertechnik GmbH
Current assignee: Wisco Lasertechnik GmbH
Priority date: 2013-02-27
Filing date: 2014-02-10
Publication date: 2016-01-06
Also published as: WO2014131491A1; DE102013101953A1

Abstract

The invention relates to a method for joining steel plates or strips (1, 2; 2') provided with a metal coating, such as an Al or Al-Si coating, to on another by means of laser-beam welding, wherein the plates or strips (1, 2; 2') are welded to one another without prior removal of the metal coating from the weld region. According to the invention, in order to economically produce welded joints of flawless quality without prior removal of the metal coating in the weld region, said welded joints being suitable in particular for subsequent hot-forming processes and press-hardening processes, the laser beam (6) is focussed in such a way that the laser beam strikes the weld region with an elongated focus (6'), wherein the elongated focus (6') extends substantially in alignment with the weld (11) and has a length that is at least 1.5 times the width of the weld. The molten phase thereby can be maintained longer, and thus the outgassing time for strength-reducing coating constituents of the plates or strips (1, 2; 2'), particularly aluminium, can be increased.

Description

A method of joining steel circuit boards or strips provided with a metallic coating

laser welding

The invention relates to a method for joining together provided with a metallic coating blanks or strips of steel by laser beam welding, in which the blanks or strips without prior removal of the

metallic coating from the weld area

be welded together. The metallic coating of the sinkers or tapes is preferably based on aluminum or an aluminum alloy.

Tailored blanks made of sheet steel (so-called tailored blanks) are used in the automotive industry to produce high

To meet requirements for crash safety with the lowest possible body weight. For this purpose, blanks or strips of different material quality and / or sheet thickness are joined together in the butt joint by laser welding. In this way, different locations of the body component can be adapted to different loads. So thicker or higher-strength steel sheet can be used in places with high load and thinner sheets or sheets of relatively soft thermoforming grades at the other locations. Such customized sheet metal blanks eliminate the need for additional reinforcing parts on the bodywork. This saves material and allows to reduce the overall weight of the body.

CONFIRMATION DPfs In recent years, boron-alloyed steels, in particular manganese-boron steels, have been developed which, during hot forming with rapid cooling, have high strengths, for example

Achieve tensile strengths in the range of 1500 to 2000 MPa. In the initial state, manganese-boron steels typically have a ferritic-pearlitic structure and have strengths of about 600 MPa. By press hardening, i. by heating up

Austenitizing temperature and then rapid

However, a martensitic microstructure can be set in the molding press, so that the steels treated in this way can reach tensile strengths in the range from 1500 to 2000 MPa.

The steel strips intended for hot forming and press hardening or tailor-made for the forming tool

Steel blanks are usually coated with a metallic coating as a corrosion protection coating to a

Scaling of the workpiece in the warm state before the

Prevent hot forming. The coating material used is usually a zinc or aluminum alloy, preferably an aluminum-silicon alloy. When welding such coated sheets, however, the problem arises that coating components,

For example, aluminum enters the weld melt and forms iron in the weld with Fe-Al compounds, which have a relatively low strength and can lead to weakening of the component in the weld or even to a component failure. In the case of a zinc coating, the problem may arise that zinc deposits on the grain boundaries during welding, as a result of which maximum tensile and compressive stresses in the weld seam can be reduced. To solve this problem, so-called stripping methods have already been proposed with which the metallic coating is removed at the edges of the sheet metal edges to be welded (see eg DE 20 2007 018 832 Ul). However, these methods are complicated and expensive.

In a method described in DE 103 24 274 AI for laser welding of aluminum-coated steel sheet, however, the aluminum coating is left in the weld area; However, it is provided to ensure a perfect weld quality that a laser beam is performed several times over the same weld. The aim here is that the components of the aluminum coating are alloyed by multiple melting in the melt of the weld seam and finely distributed therein. This known method is complex and expensive so

manufactured components considerably.

Furthermore, it is known in the art, the

Add a welding filler to weld melt to improve the strength of the weld or the manufactured component. For example, JP 07041841 A discloses a laser welding method for joining butt jointed steel sheets, in which fine melted carbon powder is supplied to the weld melt in order to achieve a carbon-rich weld melt and thus a harder weld seam.

Furthermore, from DE 10 2011 017 144 AI a method for laser beam welding is known in which a with a

metallic coating of aluminum or an aluminum alloy provided precursor of a thermoformable, press-hardenable steel is welded under inert gas, wherein the metallic coating in the formed during welding

Weld is included. For this purpose, the melt-bath dynamics of the weld metal are influenced by the use of special protective gas, so that the alloying elements of the metallic coating undergo intensive mixing with the weld metal in such a way that they become conductive

Solidification in finely divided form herein. Based on this, the present invention has the object to provide a further method of the type mentioned, can be produced with the cost of qualitatively flawless welds, the

especially suitable for subsequent hot forming and Presshärtungs- processes.

To solve this problem, a method with the

Characteristics of claim 1 proposed. Further preferred and advantageous embodiments of the invention

Method are specified in the subclaims.

The method according to the invention is characterized in that the laser beam used for joining the metallically coated blanks or strips is focused such that it strikes the weld seam area with an oblong focus, wherein the oblong focus extends substantially in line with the weld seam and has a length which is at least 1.5 times, preferably at least 1.8 times, more preferably at least 2 times its width. With regard to steel blanks provided with an aluminum-containing metallic coating, the following can be observed. Up to about 2 wt .-% aluminum is soluble in γ-iron. This means that up to this aluminum content, the steel alloy remains curable. At a

Aluminum content in the range of about 2 to 10 wt .-% is

Aluminum is soluble in the iron and is called an a-iron mixed crystal. At higher aluminum contents, intermetallic phases form.

If the aluminum content in the weld is too high, namely greater than 2% by weight, then there is a considerable or undesirable drop in hardness in the weld seam. In such cases, this is also referred to as a "hardening bag" in the weld seam, and aluminum nests and / or aluminum lancets are usually found in the weld seam

Homogenization and a high aluminum content close. In order to ensure the hot workability or hardenability of the weld, the aluminum content in the weld should therefore preferably be less than 1.8% by weight. In addition, aluminum nests and / or aluminum lancets in the weld should be prevented.

Typically, a laser optics is used in laser welding, which focuses the laser beam so that it impinges on the weld seam area with a substantially circular focus. The focus diameter in the weld region is usually in the range of 0.4 to 1.0 mm. In contrast, in the case of the laser welding method according to the invention, welding takes place with an oblong, preferably line-shaped, focus. The focus or line preferably corresponds to one substantially

rectangular or oval cross section profile (focus profile).

By combining a fiber optic cable with suitable

Fiber optic or fiber diameter and a corresponding Linienfokus.sieroptik appropriate laser beam lines or rectangular or oval foci can be generated. The im

Essentially rectangular laser beam focus can be used in the method according to the invention, for example as follows

Dimensions: 0.6 mm x 2.1 mm, 0.8 mm x 2.2 mm, 1.0 mm x 2.3 mm or 1.2 mm x 2.4 mm.

Experiments by the inventors have shown that the use of the elongated laser beam focus or line focus a significant reduction of the aluminum content in the

Achieve weld. The cause of this significant reduction in the aluminum content is seen by the inventors in the much larger Schweißbadfläche. Due to the larger area of the weld pool, significantly more time is available for outgassing at a typical welding speed

Aluminum until the solidification of the weld available. In addition, experiments carried out by the inventor have shown that when using the elongated laser beam focus or line focus, the remaining aluminum that has flowed into the weld seam is distributed very homogeneously in the weld seam. Aluminum nests and aluminum lancets are thereby avoided or at least substantially reduced. In comparison with a conventional circular laser focus with a diameter of 0.8 mm or a focus surface of about 0.50 mm ² , a line focus applied according to the invention with dimensions of, for example, 2.4 mm × 1.2 mm has a focal area of 2, 88 mm ² on. This corresponds to this

Example of an increase in area by a factor of 5.76.

A preferred embodiment of the invention

Method is accordingly characterized in that the elongated focus in the weld area has a focus area in the range of 1.2 mm ² to 3.0 mm ² .

Even the pure length comparison shows a clear

Increase in the time of solidification or outgassing; namely in relation to the above example:

2.4 mm - 0.8 mm = 1.6 mm.

This corresponds to a length increase by a factor of 3.

A preferred embodiment of the invention

The method is accordingly characterized in that the elongated focus in the weld region has a length in the

Range of 1.5 mm to 3.0 mm.

In comparison to other conceivable methods for extending the outgassing time, such as the method described in DE 103 24 274 A1, in which the laser beam is passed through the same weld seam several times around the weld seam

Melt several times, the inventive method is characterized by a significantly improved productivity. The welding speed can be greater than or equal to 5 m / min in the inventive method. For example the welding speed in the method according to the invention is in the range of 6 to 10 m / min.

With regard to the lowest possible energy consumption for welding the joined steel blanks or

Steel strips or with regard to the highest possible welding speed, a further preferred embodiment of the method according to the invention provides that the elongated focus in the weld region has a length-width ratio in the range of 2 to 4.

A further advantageous embodiment of the method according to the invention is characterized in that the with the

Laser beam generated welding a mixing of the welding bath causing filler material is added.

As a result, the homogenization of the weld pool or the weld can be further improved. The filler material can be added to the molten bath (weld pool) in the form of a powder or wire.

A further advantageous embodiment of the method according to the invention provides that the weldability generated by the laser beam and the hardenability and / or extensibility of the

Welding increasing filler material is added.

At the same time, this filler material can be so composed or supplied to the weld pool so that it causes a better mixing of the weld pool. For example, the filler material may contain at least one alloying element of a group comprising manganese, chromium, molybdenum, silicon and / or nickel. Preferably, the powdered or wire-shaped filler material has the following composition: 0.05-0.15% by weight of C

0.5-2.0% by weight of Si,

1.0-2.5% by weight of Mn,

0.5-2.0 wt% Cr + Mo, and

1.0-4.0% by weight of Ni,

Residual Fe and unavoidable impurities.

In order to prevent embrittlement of the weld seam, a further preferred embodiment of the invention provides

Procedure that the weld pool (molten bath) is applied during the laser welding with inert gas (inert gas). Pure argon, helium, nitrogen or their mixture or a mixture of argon, helium, nitrogen, carbon dioxide and / or oxygen is particularly preferably used as protective gas. ·

Hereinafter, the invention will be described with reference to a plurality

Exemplary embodiments illustrative drawing explained in more detail. They show schematically:

Fig. 1 is a perspective view of parts of a

Apparatus for carrying out the laser welding process according to the invention, wherein two substantially identical thick, press-hardenable steel blanks in

Butt joint are welded together; and

Fig. 2 is a perspective view of parts of a

Apparatus for carrying out the laser welding process according to the invention, wherein here two

different thickness, press-hardenable steel blanks are butt welded together in the butt joint. FIG. 1 shows a device with which a laser welding method according to the invention can be carried out. The device comprises a base (not shown) on which two bands or sinkers 1, 2 of steel of the same or different material quality butt along the

Joining joint 3. For example, one workpiece 1 or 2 has a relatively soft deep-drawing quality, while the other workpiece 2 or 1 consists of a high-strength steel sheet. At least one of the workpieces 1, 2 is made of press-hardenable steel, for example of manganese-boron steel. The workpieces 1, 2 and 2 'are provided with a metallic coating, preferably an Al coating or Al-Si coating. In the embodiment sketched in FIG. 1, the workpieces 1, 2 have substantially the same sheet thickness.

Above the workpieces 1, 2 a section of a laser welding head 4 is sketched, with an optics (not

shown) for supplying a laser beam and a

Focusing lens 5 is provided for the laser beam 6. Furthermore, a line 7 for supplying protective gas is arranged on the laser welding head 4. The mouth of the protective gas line 7 is essentially at the focus area of the laser beam 6 or generated with the laser beam 6

Welding bath (molten bath) 8 directed. The protective gas used is preferably pure argon or, for example, a mixture of argon, helium and / or carbon dioxide. In addition, the laser welding head 4 can be assigned a filler material supply device (9) (not shown in FIG. 1), by means of which the welding pool 8 is provided with a special additive. Material 10 is supplied in the form of a powder or wire, which is also melted by the laser beam 6 (see Fig. 2). According to the invention, the laser beam 6 is focused in such a way that it strikes the weld seam area with an elongated focus 6 ', the elongate focus 6' extending substantially flush or parallel to the weld seam 11 or to the butt joint 3. For this purpose, the laser welding head 4 is provided with a corresponding line focusing optics, for example a corresponding focusing lens 5. The elongated focus 6 'may also be referred to as a line focus.

In the example illustrated in FIG. 1, the line-shaped focus 6 'has a substantially rectangular shape

Section profile. The length of the rectangular focus 6 'is in the weld area, for example, about 1.5 mm to 3.0 mm, while its width there is about 0.6 mm to 1.2 mm. The length-width ratio of the focus 6 'is in the range of 1.5 to 4, preferably in the range of 1.8 to 4, particularly preferably in the range of 2 to 4.

Due to the line focus 6 ', the molten phase can be maintained longer and thus the outgassing time for strength-reducing coating constituents of the workpieces 1, 2, in particular aluminum can be increased.

The laser focus 6 'is guided along the joint 3 at a welding speed of, for example, about 6 to 7 m / min. The embodiment shown in Fig. 2 differs from the embodiment of FIG. 1, characterized in that the workpieces 1, 2 'are different in thickness, so that at the butt joint 3 is a thickness jump d exists. For example, one workpiece 2 'has a sheet thickness in the range of 0.5 mm to 1.2 mm, while the other workpiece 1 has a sheet thickness in the range of 1.6 mm to 2.5 mm. In addition, those in the butt joint 3 may intercommunicate

connecting workpieces 1, 2 'differ in their material quality from each other. For example, the thicker board 1 may be made of higher strength steel sheet, whereas the thinner steel board 2 'may have a relatively soft deep-drawing quality. The press-hardenable steel from which at least one of the

each other in the butt joint 3 to be joined workpieces 1, 2 and 2 ', for example, the following chemical

Composition include:

Max. 0.45% by weight of C,

Max. 0.40% by weight of Si,

Max. 2.0% by weight of Mn,

Max. 0.025% by weight P,

Max. 0.010% by weight of S,

Max. 0.8% by weight Cr + Mo,

Max. 0.05% by weight of Ti,

Max. 0.0050 wt .-% B, and

minute 0.010% by weight of Al,

Residual Fe and unavoidable impurities. As delivered, i. before a heat treatment and

rapid cooling, the yield strength is Re

press-hardenable steel plates 1, 2 and / or 2 'preferably at least 300 MPa; its tensile strength Rm is at least 480 MPa, and its elongation at break Aeo is at least 10%. After hot forming, ie austenitizing at about 900 to 920 ° C. and subsequent rapid cooling, these steel blanks have a yield strength Re of about 1100 MPa, a tensile strength Rm of about 1,500 to 2,000 MPa and an ultimate elongation Aeo of about 5 , 0% up.

The used powder or wire-shaped filler material 10 has, for example, the following chemical composition:

0.1% by weight of C,

0.8% by weight of Si,

1.8% by weight of Mn,

0.35% by weight Cr,

0., 6 wt .-% Mo, and

2.25% by weight of Ni,

Residual Fe and unavoidable impurities.

The manganese content of the filler material 10 is preferably higher than the manganese content of the press-hardenable workpieces 1, 2 or 2 '. For example, the manganese content of the

Additional material 10 by about 0.2 wt .-% higher than the manganese content of the press-hardened workpieces 1, 2 or 2 '. Furthermore, it is favorable if the content of chromium and molybdenum of the filler material 10 is higher than in the press-hardenable workpieces 1, 2 or 2 '. Preferably, the combined chromium-molybdenum content of the filler material 10 is about 0.2 wt .-% higher than the combined chromium-molybdenum content of the press-hardenable workpieces 1, 2 and 2 '. The nickel content of the filler material 10 is preferably in the range of 1 to 4 wt .-%. In addition, the filler material 10 preferably a lower carbon content than the press-hardenable steel of the workpieces 1, 2 and 2 '.

The embodiment of the present invention is not limited to the embodiments illustrated in the drawings

limited. On the contrary, further variants are conceivable which make use of the invention specified in the appended claims even in the case of designs deviating from these exemplary embodiments. For example, the

Laser beam are also focused so that the elongated focus 6 'in the weld area has a substantially oval or polygonal cross-sectional profile (focus profile).

Claims

P a n t a n s p r e c h e

A method of joining metallic coated circuit boards or strips (1, 2, 2 ') of steel by laser welding, in which the

Blanks (1, 2, 2 ') are welded together without prior removal of the metallic coating from the weld area, characterized in that the laser beam (6) is focused so that it impinges with oblong focus (6') on the weld area , wherein the elongated focus (6 ') in

Extends substantially in line with the weld (11) and has a length which is at least 1.5 times its width.

A method according to claim 1, characterized in that the laser beam (6) is focused such that the elongated focus (6 ') in the weld area in the

Essentially rectangular, oval or polygonal

Focus profile has.

A method according to claim 1 or 2, characterized in that the elongated focus (6 ') in the weld area a length-width ratio in the range of 1.5 to 4

having .

Method according to one of claims 1 to 3, characterized in that the elongated focus (6 ') in the welding seam area has a length in the range of 1.5 mm to 3.0 mm.

Method according to one of claims 1 to 4, characterized in that the elongated focus (6 ') in

Weld area a focus area in the range of

1.2 mm ² to 3.0 mm ² .

Method according to one of claims 1 to 5, characterized in that the laser focus (6 ') with a

Welding speed in the range of 5 to 10 m / min along the joint (3) is guided.

Method according to one of claims 1 to 6, characterized in that with the laser beam (6)

produced a thorough mixing of the

Welding bath (8) causing additional filler (10) is added.

Method according to one of claims 1 to 7, characterized in that the hardening and / or extensibility of the weld (11) increasing filler material (10) is added to the welding bath (8) produced with the laser beam (6).

Method according to one of claims 1 to 8, characterized in that the metallic coating of the steel plates to be welded together or

Steel bands (1, 2, 2 ') an aluminum or a

Aluminum alloy-containing metallic coating is.

10. The method according to any one of claims 1 to 9, characterized in that at least one / one of the steel plates to be joined or steel strips (1, 2, 2 /) consists of thermoformable, curable by rapid cooling steel.

11. The method according to any one of claims 1 to 10, characterized

characterized in that the weld pool (8) during the

Laser welding is applied with inert gas.

12. The method according to claim 11, characterized in that as protective gas pure argon or a mixture of argon and carbon dioxide is used.