METHOD FOR WELDING COATED STEEL SHEETS

Abstract

A method for welding coated steel sheets, particularly steel sheets that are coated with an aluminum-silicon metallic coating layer, is provided. A configuration of two laser beams is provided, wherein the laser beams act on a weld pool that is to be formed, at least one laser beam rotates around a rotation axis so that the laser beams execute a movement relative to each other, and the laser beams are guided along a welding axis. In order to achieve a mixing of the weld pool, a defined stirring effect and a defined welding speed in relation to each other are adhered to, wherein a mathematically defined condition applies to the stirring effect.

Claims

1-15. (canceled)

16. A method for welding coated steel sheets, comprising the steps of: providing a configuration (1, 11, 12) of first and second laser beams (2, 3), wherein the laser beams act on a weld pool that is to be formed, at least one laser beam (3) rotates around a rotation axis (5) so that the laser beams (2, 3) execute a movement relative to each other, the laser beams (2, 3) are guided along a welding axis (4); and achieving a mixing of the weld pool by adhering to a defined stirring effect and a defined welding speed in relation to each other are adhered to, wherein the following condition applies to the stirring effect (η): $\eta =\frac{f_{\mathrm{rot}}}{v_w}$ where f.sub.rot is the rotation frequency and v.sub.w is the welding speed and wherein the following conditions apply: $4\le \eta \le \frac{120}{v_w}\left[\frac{1}{\mathrm{mm}}\right]$ $\mathrm{and}$ $4\le v_w\le 14\left[\frac{m}{\min }\right].$

17. The method according to claim 16, further comprising the steps of a) positioning the laser beams (2, 3) symmetrically around a rotation axis (5) and rotating the laser beams (2, 3) around the rotation axis (5) in diametrically opposed positions, or b) guiding the first laser beam (2) along a welding axis and rotating the second laser beam (3) around the first laser beam (2), or c) rotating the first laser beam (2) with a first smaller radius around the rotation axis (5) and rotating the second laser beam (3) with a larger radius around the rotation axis (5), or d) a combination of a), b) and/or c).

18. The method according to claim 16, further comprising the step of rotating the laser beams (2, 3) symmetrically relative to projected areas or spots, wherein the laser beams (2, 3) are each spaced apart from a spot center by the spot spacing x.sub.df, each laser beam has a diameter df of 0.1 mm to 1 mm, and the laser beams (2, 3) have a total coverage width defined by a sum of a spacing of the spot centers from each other plus one spot diameter, wherein the total coverage width is between 0.5 mm and 2.5 mm.

19. The method according to claim 18, wherein the spot spacing x.sub.df≥0.8*d.sub.f.

20. The method according to claim 17, wherein in the configuration (11), the first and second laser beams (2, 3) are positioned orbitally, the first laser beam (3) remains along a weld advancing direction (10) on a central axis of the weld pool, while the second laser beam (3) rotates around a rotation axis (5), and the rotation axis (5) lies on a welding axis (4) or oscillates around the welding axis (4) and constitutes the spot center of the first spot (2).

21. The method according to claim 19, wherein the spot diameter is between 0.1 and 1 mm and the following conditions apply: $x_{\mathrm{df}}\ge 0.8\star d_f\mathrm{and}$ $0.45\mathrm{mm}\le x_{\mathrm{df}}+\frac{d_f}{2}\le 1.5\mathrm{mm}$

22. The method according to claim 17, wherein in the configuration (12), two laser beams (2, 3) rotate around the rotation axis (5), the first laser beam (2) rotates with a first radius around the rotation axis (5), the second laser beam (3) rotates with a second radius around the rotation axis (5), one of the first radius and the second radius is greater than the other, and the following conditions apply: $0.45\mathrm{mm}\le x_{\mathrm{df}}-x_{\mathrm{off}}+\frac{d_f}{2}\le 1.5\mathrm{mm}$ $x_{\mathrm{df}}\ge 0.8\star d_f$ $0<x_{\mathrm{off}}<\frac{x_{\mathrm{df}}}{2}$

23. The method according to claim 16, wherein the welding is performed with a laser power of between 2 and 10 kW.

24. The method according to claim 16, wherein the stirring effect ii in mm.sup.−1 is between 4 and 30 mm.sup.−1.

25. The method according to claim 16, wherein the welding speed v.sub.w is between 5 and 12 m/min.

26. The method according to claim 16, wherein at least one of the coated steel sheets comprises a boron-manganese steel base material which can be hardened by means of an austenitization and quenching process to a tensile strength of greater than 900 MPa.

27. The method according to claim 16, wherein at least one of the coated steel sheets comprises a steel base material having the following alloy composition in % by mass: TABLE-US-00003 carbon (C) 0.03-0.6  manganese (Mn) 0.3-3.0 aluminum (Al) 0.01-0.07 silicon (Si) 0.01-0.8  chromium (Cr) 0.02-0.6  nickel (Ni) <0.5 titanium (Ti) 0.01-0.08 niobium (Nb) <0.1 nitrogen (N) <0.02 boron (B) <0.02 phosphorus (P) <0.01 sulfur (S) <0.01 molybdenum (Mo) <1 residual iron and smelting-related impurities.

28. The method according to claim 16, wherein at least one of the coated steel sheets comprises a steel base material having the following alloy composition in % by mass: TABLE-US-00004 carbon (C) 0.03-0.36 manganese (Mn)  0.3-2.00 aluminum (Al) 0.03-0.06 silicon (Si) 0.01-0.20 chromium (Cr) 0.02-0.4  nickel (Ni) <0.5 titanium (Ti) 0.03-0.04 niobium (Nb) <0.1 nitrogen (N) <0.007 boron (B) <0.006 phosphorus (P) <0.01 sulfur (S) <0.01 molybdenum (Mo) <1 residual iron and smelting-related impurities.

29. A method of preparing a sheet bar comprising a first steel sheet and a second steel sheet, wherein the first steel sheet and the second steel sheet are welded to each other according to the following steps: providing a configuration (1, 11, 12) of first and second laser beams (2, 3), wherein the laser beams act on a weld pool that is to be formed, at least one laser beam (3) rotates around a rotation axis (5) so that the laser beams (2, 3) execute a movement relative to each other, the laser beams (2, 3) are guided along a welding axis (4); and achieving a mixing of the weld pool by adhering to a defined stirring effect and a defined welding speed in relation to each other are adhered to, wherein the following condition applies to the stirring effect (η): $\eta =\frac{f_{\mathrm{rot}}}{v_w}$ where f.sub.rot is the rotation frequency and v.sub.w is the welding speed and wherein the following conditions apply: $4\le \eta \le \frac{120}{v_w}\left[\frac{1}{\mathrm{mm}}\right]$ $\mathrm{and}$ $4\le v_w\le 14\left[\frac{m}{\min }\right].$

30. The method according to claim 29, wherein the first and second steel sheets have different alloy compositions.

31. The method according to claim 29, wherein at least one of the first and second steel sheets comprises a coated steel sheet.

32. The method according to claim 29, wherein at least one of the first and second steel sheets comprises a steel base material having the following alloy composition in % by mass: TABLE-US-00005 carbon (C) 0.03-0.36 manganese (Mn)  0.3-2.00 aluminum (Al) 0.03-0.06 silicon (Si) 0.01-0.20 chromium (Cr) 0.02-0.4  nickel (Ni) <0.5 titanium (Ti) 0.03-0.04 niobium (Nb), nitrogen(N, boron(B),  0.0-0.16 phosphorus (P) and sulfur (S) together molybdenum (Mo) <1 residual iron and smelting-related impurities.

33. A method of preparing a sheet bar comprising a first steel sheet and a second steel sheet, wherein the first steel sheet and the second steel sheet are welded to each other according to the following steps: providing a configuration (1, 11, 12) of first and second laser beams (2, 3), wherein the laser beams act on a weld pool that is to be formed, at least one laser beam (3) rotates around a rotation axis (5) so that the laser beams (2, 3) execute a movement relative to each other, the laser beams (2, 3) are guided along a welding axis (4); and achieving a mixing of the weld pool by adhering to a defined stirring effect and a defined welding speed in relation to each other are adhered to, wherein the following condition applies to the stirring effect (η): $\eta =\frac{f_{\mathrm{rot}}}{v_w}$ where f.sub.rot is the rotation frequency and v.sub.w is the welding speed and wherein the following conditions apply: $4\le \eta \le \frac{120}{v_w}\left[\frac{1}{\mathrm{mm}}\right]$ $\mathrm{and}$ $4\le v_w\le 14\left[\frac{m}{\min }\right];$ wherein at least one of the first and second steel sheets comprises a steel base material having the following alloy composition in % by mass: TABLE-US-00006 carbon (C) 0.03-0.36 manganese (Mn)  0.3-2.00 aluminum (Al) 0.03-0.06 silicon (Si) 0.01-0.20 chromium (Cr) 0.02-0.4  nickel (Ni) <0.5 titanium (Ti) 0.03-0.04 niobium (Nb), nitrogen(N, boron(B),  0.0-0.14 phosphorus (P) and sulfur (S) together molybdenum (Mo) <1 residual iron and smelting-related impurities.

34. The method according to claim 33, wherein the first and second steel sheets have different alloy compositions.

35. The method according to claim 33, wherein at least one of the first and second steel sheets comprises a coated steel sheet.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] The invention will be explained based on the drawings. In the drawings:

[0058] FIG. 1: schematically depicts the symmetrical, asymmetrical, and orbital rotation of the welding laser beams;

[0059] FIG. 2: shows the process window according to the invention as it relates to the stirring effect;

[0060] FIG. 3: shows the depiction from FIG. 2 with the meaning of the outlying regions provided;

[0061] FIG. 4: is a table showing 16 different tests in the comparison of embodiments that are according to the invention and embodiments that are not according to the invention;

[0062] FIG. 5: depicts of a symmetrical stirring apparatus with the functions of the spot spacing and the spot diameter relative to each other;

[0063] FIG. 6: shows the process window with a symmetrical stirring apparatus;

[0064] FIG. 7: is a schematic depiction of an orbital stirring apparatus with the functions of the spot spacing and the spot diameter;

[0065] FIG. 8: shows the process window of the orbital stirring apparatus;

[0066] FIG. 9: depicts an asymmetrical stirring apparatus with the functions of the spot spacing, the spot diameter, and the eccentricity;

[0067] FIG. 10: shows a hardened weld seam in a polished micrograph depiction according to test T1 in the table;

[0068] FIG. 11: shows a polished micrograph according to test T2 in the table;

[0069] FIG. 12: shows a polished micrograph of the weld seam according to test T4 in the table;

[0070] FIG. 13: shows a cross-section through a weld seam in a polished micrograph according to test T16 in the table.

DETAILED DESCRIPTION OF THE INVENTION

[0071] FIG. 1 shows three different principally possible and also mutually combinable laser beam configurations, wherein in the laser beam configurations shown, with a symmetrical configuration (FIG. 1a), the laser beams are positioned symmetrical to a rotation axis and in this case, are rotated at diametrically opposed positions around the rotation axis.

[0072] The symmetrical configuration can advantageously achieve the maximum stirring effect.

[0073] With an asymmetrical apparatus (FIG. 1b), one laser beam is positioned closer to the rotation axis than the other so that an eccentricity is produced. The asymmetrical apparatus advantageously makes it possible to influence the desired weld seam geometry.

[0074] With the orbital apparatus (FIG. 1c), a central laser beam is provided, which is moved along a weld advancing direction while the second laser beam, which is spaced apart from this central one, rotates around it and around the rotation axis.

[0075] The orbital apparatus can advantageously have a compensating effect on possible differences in sheet thickness.

[0076] FIG. 5 shows the symmetrical stirring apparatus in greater detail.

[0077] With this laser beam configuration 1, there are two laser beams 2, 3, which are each spaced about the same distance apart from an idealized weld pool center 4. Preferably, the ideal weld pool center 4 also coincides with the rotation axis 5 around which the two laser beams 2, 3 rotate in accordance with the rotation directions 6, 7. Accordingly, sample sequential positions 2′, 3′ that are offset by 90° are shown. The laser beams 2, 3 or more precisely, their projected areas (spots) have a given diameter d.sub.f corresponding to the expansion arrows 8, 9.

[0078] The two laser beams 2, 3 or more precisely, their projected areas (spots), viewed from the center, are respectively spaced apart by the spot spacing x.sub.df. The theoretical weld pool width thus equals the spot spacing plus one half of each spot diameter. The weld advancing movement takes place in accordance with the arrow 10 along the idealized weld pool center 4 at a weld advancing speed v.sub.w.

[0079] With this configuration of a symmetrically rotating apparatus, the spot diameter df preferably lies in a range from 0.1 to 1 mm.

[0080] The spacing of the spot centers from each other plus the spot diameter is preferably between 0.5 mm and 3 mm, in particular from 0.9 mm to 2.5 mm, wherein for the spot spacing x.sub.df, preferably the following condition applies: x.sub.df≥0.8*d.sub.f.

[0081] FIG. 6 shows a suitable process window for the symmetrical stirring apparatus with regard to the relationship between the spot spacing and diameter. As explained above, the spot diameter df is as a rule between 0.1 and 1 mm.

[0082] In another advantageous laser beam configuration 11 (FIG. 7), two laser beams 2, 3 are orbitally positioned, which means that a first spot 2, in accordance with the weld advancing direction 10, remains on the welding axis 4 while a second spot 3 rotates around a rotation axis 5, which lies on the welding axis 4 and constitutes the center point of the first spot 2.

[0083] The rotation of the second spot 3 correspondingly occurs along the rotation direction 7, which is positioned at a particular radius around the rotation axis 5. By way of example in FIG. 7, the different positions of the second laser spot 3 are shown here as rotated by 180° with the position 3′. But full rotations are executed during the welding along the weld advancing direction 10.

[0084] The welding axis 4 simultaneously also constitutes the idealized weld pool center 4.

[0085] In this advantageous embodiment, the spot diameter likewise is between 0.1 and 1 mm, wherein the following condition applies here: 0.45 mm≤x.sub.df+d.sub.f/2≤1.5 mm.

[0086] The condition x.sub.df≥0.8*d.sub.f also applies here. FIG. 8 shows the process window that applies to the orbital stirring apparatus in accordance with the above-mentioned basic conditions, wherein the function of the spot spacing over the spot diameter is indicated in the process window and the corresponding region according to the invention is located within the enclosed area.

[0087] In another advantageous embodiment of a laser beam configuration 1 (shown in FIG. 9), two laser spots 2, 3 once again rotate around a rotation axis 5, but a first rotation direction 6 of a first laser beam 2 or first laser spot 2 is positioned closer to the rotation axis than the second rotation direction 7 of the second laser beam 3. The spot spacing center is thus spaced apart from the weld pool center 4 or more precisely, is positioned offset from it.

[0088] In this advantageous embodiment, the spot diameter df is once again between 0.1 and 1 mm, wherein for this, the following conditions are additionally met:

[00005]$0.45\mathrm{mm}\le x_{\mathrm{df}}-x_{\mathrm{off}}+\frac{d_f}{2}\le 1.5\mathrm{mm}$$x_{\mathrm{df}}\ge 0.8\star d_f$$0<x_{\mathrm{off}}<\frac{x_{\mathrm{df}}}{2}$

[0089] FIG. 2 shows the process window according to the invention with regard to the stirring effect. In this connection, FIG. 3 shows the respective effects when the process is carried out with unsuitable parameters, i.e. ones that lie outside the process window.

[0090] The selection of an excessively powerful stirring effect in combination with a high welding speed can result in humping (unstable welding process), increased occurrence of spattering, and even perforation of the laser weld seam.

[0091] Surprisingly, even with a stirring effect that is too weak, the spattering tendency can increase sharply.

[0092] A laser welding speed (v.sub.w) of less than 4 m/min is in fact technically possible, but is no longer worthwhile economically.

[0093] The table in FIG. 4 shows sixteen welding tests; the welding tests were performed with different weld advancing speeds, different stirring effects, and different power levels. After the hardening, the weld seams were examined and classified according to their weld seam homogeneity and process stability. The abbreviation “n.a.” stands for “not assessable” since in these tests, a stable weld seam could not be produced.

[0094] FIG. 10 shows a weld seam structure after the hardening (example T1 from the table in FIG. 4) in which the parameters according to the invention were not adhered to. Based on visual inspection alone, the weld seam structure clearly lacks homogeneity after the hardening; the weld advancing speed here was 6 m per minute. The spot diameter was 0.3 mm, but with a spot spacing of 0, which means that only a single laser was used. It is clear that with this conventional method, it is not possible to achieve a qualitatively satisfactory result.

[0095] FIG. 11 shows the result of an embodiment according to the invention (example test 2 from the table in FIG. 4); the polished micrograph after the hardening is homogeneous. The spot diameter here was 0.3 mm; a symmetrical stirring apparatus was used in which the spot spacing was 0.9 mm.

[0096] The laser power was 4.3 kW and the weld advancing speed was 6 m per minute. The stirring effect η was 4.125 mm.sup.−1, the stirring effect being the quotient of the rotation frequency and the welding speed or more precisely the weld advancing distance.

[0097] The distance of the spot center from the rotation axis was 0.45 mm, which means that the spots orbited the rotation axis on a radius.

[0098] FIG. 12 shows test 4, which is not according to the invention. The spot diameter and the spot spacing do lie within a range according to the invention as does the weld advancing speed, which at 6 m per minute corresponds to that of test 2, and the distance of the spot center from the rotation axis is indeed also the same, but the stirring effect, as the quotient of the rotation frequency and weld advancing speed, is too weak so that the polished micrograph after the hardening exhibits a clear lack of homogeneity.

[0099] FIG. 13 shows the result of test 16 according to the invention. It is clear that a homogeneous weld seam structure is present. The spot spacing in this case was 0.4 mm with a spot diameter of 0.3 mm, wherein the advancing speed corresponded to that of the other tests. The stirring effect η, at 4.125 mm.sup.−1, lies within the range according to the invention; the distance of the spot center from the rotation axis was 0.2 mm.

[0100] With the invention, it is advantageous that through a specific selection of parameters and a corresponding process control, homogeneous weld seams can be reliably achieved when welding aluminum-silicon-coated sheets.