Methods for process-reliable laser welding and use of the method

Abstract

The invention relates to a method for the reliable laser welding of high-strength steels (1), wherein hydrogen-containing impurities (3) are reduced or removed by thermal treatment. The method according to the invention is characterized in that the impurities (3) are removed by heating the areas of the steels (1) to be welded, after which the laser weld seam (6) is immediately produced. The method can preferably be used for welding sheet metal (1) in car body construction.

Description

[0001] The invention relates to a method for the reliable laser welding of high-strength steels according to the type defined in the preamble of claim 1. Furthermore, the invention relates to the use or application of such a method.

[0002] So-called high-strength or ultra-high-strength steels, which have a tensile strength above 800 MPa and exhibit good cold formability, are becoming increasingly important as body steels in automotive construction. They offer significant potential for reducing vehicle weight. This, in turn, leads to lower energy consumption and thus fuel and electricity savings, which ultimately result in lower pollutant emissions. Such steels are typically referred to as Advanced High-Strength Steels (AHSS). The third generation of such steels plays a particularly crucial role here, as they combine good cold formability with maximum strength and good weldability.

[0003] Laser welding plays a central role in welding such steels, for example for welding body parts in vehicle construction, as it enables increased accessibility even for complex geometries and a high process speed that can be adapted to each individual case, so that overall decisive advantages in terms of production time and thus corresponding cost advantages can be achieved.

[0004] In practice, steels used in the welding process, for example in automotive engineering, come from previous processes, such as cold forming. The steels therefore have corresponding impurities on their surface, which may include, for example, water- or oil-based impurities. Such impurities typically contain hydrogen. The heat input during laser welding can cause the hydrogen contained in the impurities to break down. The resulting hydrogen ions or protons can become embedded in the welded material. This can lead to increased cold cracking after the weld has cooled.

[0005] In principle, this risk posed by hydrogen to the quality of welds has long been known. For example, patent application DD 257 088 A1 describes a method for preventing hydrogen-induced cold cracking when welding workpieces made of high-strength steel. It describes that, if possible, a defined annealing process should be carried out immediately after the exposure to protons, in the form of hydrogen ions, that occurs during welding in order to remove the moderately bound hydrogen from the workpieces through annealing.

[0006] The problem is that this is comparatively time-consuming, as the time required for the hydrogen to diffuse out depends on the workpiece size and can be correspondingly long for larger workpieces. Furthermore, this time must be laboriously calculated based on known diffusion coefficients and maintained for each workpiece.

[0007] In the context of assembly within a process chain, the application of this strategy therefore represents a significant disadvantage, as very large furnaces would be required, especially for vehicle bodies, and the time the parts would have to remain in the furnace far exceeds the cycle times of conventional series production. Furthermore, the large furnaces require a significant energy input, which would lead to correspondingly high thermal losses and thus high emissions during vehicle production.

[0008] The object of the present invention is therefore to provide an improved method for process-reliable laser welding of high-strength steels.

[0009] According to the invention, this object is achieved by a method having the features in claim 1, and here in particular in the characterizing part of claim 1.

[0010] Further advantageous embodiments and refinements of the method are disclosed in the dependent subclaims. A particularly preferred use of the method is specified in claim 6. Here, too, an advantageous embodiment is disclosed in the dependent subclaim.

[0011] The method according to the invention provides for the removal of hydrogen-containing impurities by heating the areas of the steel to be welded, after which the laser weld seam is created immediately. Heating is therefore not carried out across the entire workpiece, but only in those areas where welding is to take place. Laser welding then takes place immediately. "Immediate" means that there are no unnecessary waiting times, so that, for example, in a process chain, the two steps follow one another directly or with intermediate positioning and clamping of the steel.

[0012] In contrast to the prior art mentioned at the beginning, the hydrogen is not heated out again from this area after the weld has been created, but rather a targeted heating process is carried out to thermally clean the areas of the steel to be welded. During this heating process, which according to a very advantageous development can be carried out to a temperature of more than 500°C, the contaminants such as water, oil or dirt are evaporated so that they are no longer present or only present to a very small extent in the area of ​​the subsequent weld seams. This makes it possible to prevent the risk of unwanted cold cracking and thus very detrimental damage and weakening of the weld seams. This can be integrated very efficiently into production under the process conditions that occur, for example, in vehicle body construction.

[0013] As already mentioned above, after heating, i.e., thermally cleaning the areas to be welded, they can be positioned and / or clamped before the laser welding itself begins. However, unnecessary delays such as intermediate storage of the partially cleaned workpieces should be avoided to prevent further contamination. Welding should therefore take place immediately after cleaning, as defined above.

[0014] The heating itself can be carried out using an induction treatment or, alternatively, using a laser beam. This allows the required temperature, preferably more than 500°C, to be reached simply and efficiently in a short time in order to largely or ideally completely remove the hydrogen-containing contaminants from the areas to be welded.

[0015] The process according to the invention is ideally suited for welding high-strength steels, as already explained above. In particular, the process is intended for use in welding high-strength or ultra-high-strength steels of the third generation, also known as Advanced High Strength Steel, 3rd Gen.

[0016] A particularly advantageous further development of this application provides for the method to be used for the lap welding of two metal sheets, in particular body panels, while maintaining a degassing gap. In particular, with such lap welding of metal sheets, such as body panels, four sides are involved in the weld seam, i.e., the two opposite sides of one and the other sheet. Efficiently cleaning these four sides is difficult. However, the method according to the invention achieves this reliably and efficiently by heating the areas to be welded later for thermal cleaning.

[0017] Further advantageous embodiments of the method and its use also emerge from the exemplary embodiment, which is described in more detail below with reference to the figures.

[0018] Showing: Fig. 1 a schematic representation of the hydrogen input during welding of contaminated sheets; Fig. 2 a schematic representation of the overlap welding of two sheets according to the prior art; Fig. 3 two ways to clean the areas to be welded by heating; and Fig. 4 a representation analogous to that in Fig. 2 in the method according to the invention.

[0019] In the presentation of the Fig. 1 shows a sheet of metal, labeled 1, for example, made of a high-strength third-generation steel. On its surface, although this is only shown schematically on the surface shown above, there is a zinc layer labeled 2, on which contaminants labeled 3 can be seen. These can be, for example, a thin film of oil, water, dirt, and / or the like. Typical for such contamination based on water or organic chemical compounds is that they contain hydrogen. During the welding process, which is carried out here by the indicated laser beam 4 in the feed direction V from right to left, hydrogen ions H are formed from these contaminants. +released, which accumulate in the area of ​​the laser beam 4 and thus penetrate into the material to be welded. The accumulation is designated by I, the penetration into the material by II. Different concentrations of hydrogen ions H then form in the material, as designated by III. + over the height of the subsequent weld seam, whereby a portion of the hydrogen ions, as indicated by the number IV, also recombines and diffuses out again as hydrogen gas H2. A portion of the hydrogen ions H + However, it will always remain within the material in varying concentrations throughout the weld seam. This then leads to the risk of increased cold cracking.

[0020] This within the framework of Fig. 1 theoretically considered case now occurs very frequently in practice, for example when body panels made of high-strength steel are welded together in an overlap. In the illustration of the Fig. 2 shows two such sheets 1.1 and 1.2, which are also designated 1. Like sheet 1, they consist of Fig. 1 made of a high-strength steel material, a zinc layer 2 surrounding the surface and the impurities 3 distributed over the entire surface of the two sheets 1. The two sheets 1 are here provided with a degassing gap designated 5 between the sheets 1 in the illustration of the Fig. 2 are positioned on the left and ideally clamped together, although this is not explicitly shown here. This shows the arrangement before welding.

[0021] In the presentation of the Fig. 2 shows the same structure after welding. A weld seam designated 6 connects the two sheets 1 while maintaining the degassing gap 5. In the area where the weld seam 6 has been drawn, both the zinc layer 2 and the contamination 3 have been removed, which occurred due to the thermal effects during welding. The hydrogen ions or protons H formed in this process + However, as shown in the presentation of the Fig. 1, penetrated into the material of the two sheets 1 and lead to an increased risk of cold cracking in the area of ​​the weld seam 6.

[0022] To prevent this, it is now possible, as shown in the presentation of the Fig. 3 is schematically indicated, it is provided that the areas to be welded are heated to temperatures of more than 500° C. In the illustration of the Fig. 3 on the left, the heating by induction is shown schematically by indicated induction coils 7, in the illustration of the Fig. 3 right, the heating by means of two laser beams 8 provided for heating. After this thermal cleaning, the sheets 1 are clamped together in complete overlap while maintaining the degassing gap 3, as is again analogous to the illustration in Fig. 2 left in the illustration of the Fig. 4 on the left. The difference now is that the areas of the overlap weld intended for welding are completely free of the zinc layer 2 and the contaminants 3. Ideally, heating using the induction coils 7 or the laser beams 8 is carried out to such a high temperature and for such a long time that all contaminants and the zinc are reliably evaporated. However, it can also be a decisive advantage if only a large portion of these materials can be thermally removed during heating.

[0023] Analogous to the representation in Fig. 2, the weld seam 6 is then drawn over the laser steel 4 (not shown here) to weld the sheets 1. In contrast to the illustration in Fig. 2, however, there is the decisive advantage that this weld is free or largely free of hydrogen ions or protons H +so that only a minimal risk of cold cracking remains in the area of ​​weld 6.

[0024] The process can be easily, efficiently and quickly integrated into conventional processes and ensures high-quality, process-reliable welding, which can be used in particular for welding sheet metal 1 in body construction in the automotive industry. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DD 257 088 A1

[0005]

Claims

[1] Process for the reliable laser welding of ultra-high-strength steels (1), wherein hydrogen-containing impurities (3) are reduced or removed by a thermal treatment, characterized by that the impurities (3) are removed by heating the areas of the steels (1) to be welded, after which the laser weld seam (6) is immediately produced. [2] Method according to claim 1, characterized by that the heating takes place to a temperature of more than 500° C. [3] Method according to claim 1 or 2, characterized by that the steels (1) are positioned and clamped after heating and before the laser weld seam (6) is created. [4] Method according to claim 1, 2 or 3, characterized by that heating is carried out by induction (7). [5] Method according to one of claims 1, 2 or 3, characterized by that the heating is carried out by means of at least one laser beam (8). [6] Use of the method according to one of claims 1 to 5 for welding high-strength steels, in particular high-strength steels of the third generation. [7] Use according to claim 6, characterized by that two sheets (1) are welded by means of an overlap weld while maintaining a degassing gap (5).

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