Method for determining the joining suitability of a sheet metal component for vehicle construction and joining system

Abstract

The invention relates to a method for determining the suitability of a sheet metal component for joining in vehicle construction, comprising the steps of: - clamping sheets (A, B) while maintaining a required degassing gap; - welding, in particular laser welding, of the sheets (A, B) with distributed energy so that both sheets (A, B) are completely welded through at the weld seams; - determining the cold cracking tendency of the sheets (A, B); and - deriving the suitability of the sheet metal component for joining from the determined cold cracking tendency of at least one of the sheets (A, B). According to the invention, a measurement of the diffusible hydrogen content in a weld seam is carried out, and a cold cracking probability is determined based on the determined hydrogen content to ascertain the suitability for joining. The invention further relates to a joining system for operating such a method.

Description

[0001] The invention relates to a method for determining the joining suitability of a sheet metal component for vehicle construction according to the preamble of patent claim 1. Furthermore, the invention relates to a joining system for operating such a method.

[0002] High-strength body steels with a strength of at least 800 MPa are increasingly being used in modern car body construction, as their high strength and good formability offer great potential in lightweight automotive construction. However, these materials are susceptible to cracking phenomena such as hot cracking, cold cracking, and liquid metal embrittlement. A typical problem occurs at the end of a weld, where a hot crack can form in an end crater, which in turn promotes the formation of a cold crack. This cold crack can propagate along the entire weld and significantly weaken the structure.

[0003] The tendency to cold cracking increases significantly, particularly when processing high-strength steels (AHSS). Since conventional base material information does not provide reliable information on joining suitability or susceptibility to cold cracking, a special testing method is necessary to evaluate materials and welding parameters with regard to stress-induced hydrogen embrittlement during the welding process. Current testing methods, such as tensile tests, only subsequently record the load on the welds and do not provide information on the critical stresses that occur during welding. An investigation into the influence of external loads on hydrogen embrittlement in laser welds has not yet been sufficiently developed, although this is crucial for preventing cold cracking during the welding process.

[0004] DE102023002714A1 already discloses a method for determining the cold cracking tendency of a sheet metal component, comprising the following steps: clamping sheets while maintaining a required degassing gap; welding the sheets in complete overlap with a defined heat input per unit length so that both sheets are completely welded through; determining the cold cracking tendency by optical inspection at predetermined time intervals and by determining the crack growth of a cold crack over these intervals; and deriving the joining suitability of the sheet metal component based on the determined cold cracking tendency of at least one of the sheets.

[0005] The object of the invention is to further develop a test methodology or a test procedure with appropriate test specimen and welding parameters in such a way that a tendency to cold cracking and / or hydrogen embrittlement and consequently a weldability of a sheet metal component is detected at an early stage.

[0006] This object is achieved by means of a method having the features of patent claim 1 and by means of a joining system according to the invention. Advantageous refinements of the joining system according to the invention are to be regarded as advantageous embodiments of the method according to the invention, wherein the means of the joining system are used to carry out the method steps. Furthermore, advantageous developments of the invention are described by the dependent claims, the following description, and the figures.

[0007] One aspect of the invention relates to a method for determining the joining suitability of a sheet metal component for vehicle construction, comprising the steps: - Clamping of sheets while maintaining a required degassing gap; - Welding, in particular laser welding, of the sheets, e.g. in complete overlap with heat energy, so that both sheets are completely welded through; - Determination of cold cracking tendency by visual inspection of the sheets; and - Deriving the joining suitability of the sheet metal component from the determined cold cracking tendency of at least one of the sheets.

[0008] Therefore, in a first step, the sheets are clamped against each other and clamped against a base. The sheets are thus fixed to a base and can be welded together, with the sheets spaced a certain distance apart to allow for a degassing gap. This spacing is preferably achieved using a spacer film.

[0009] The next step involves welding the sheets, particularly by laser welding. The resulting weld joins the two sheets together after the welding process is completed by continuously combining molten material from the first sheet with the weld metal and molten material from the second sheet.

[0010] The welding process can be performed under the influence of oil, but the oil can also be omitted. It is advantageous to conduct tests with and without oil in parallel to obtain a broader test range.

[0011] The weld is then tested for cold cracking by examining the resulting sample at specified intervals. In the initial step, the inspection is preferably carried out using slightly magnified visual aids, but can be supported by higher magnification devices such as microscopes. X-ray equipment for non-destructive testing and ultrasound examinations are also possible. The intervals should increase as time passes, so that shorter intervals are initially chosen to investigate early crack propagation, which can increase over time. These cracks are best documented to allow for subsequent reconstructing of the crack propagation behavior. For better documentation, it is advisable to take photographs of the cold cracks.

[0012] The temporal development of the cold cracks can then be used to determine the joining suitability of one or both sheets, and thus of the sheet metal component. The more pronounced the cold cracks are, the poorer the joining suitability. This relationship can be quantified using empirical data or image recognition methods.

[0013] This method is used to test cold crack susceptibility during welding, preferably during laser welding of high-strength body steels. A special specimen geometry for a test procedure, as well as the test procedure itself, are provided. This technical solution for the early detection of delayed crack formation leads to increased process stability and product quality.

[0014] An advantageous effect of the invention is that it creates a defined, standardized, and repeatable procedure for testing the weldability of a steel component. Furthermore, the testing method allows for the evaluation of the influence of different parameters, and early detection with simple geometry is possible. This enables rapid classification and ranking of the crack propensity of different materials. Such test methods can be used for early verification of weldability, e.g., in laser welding of crash-relevant components such as battery frames or overlapping components in the bodyshell. As a result, process stability and product quality are improved when laser welding high-strength steel materials. Early detection of the risk of cracking lessens the need for rework and reduces potential process instabilities.

[0015] To achieve the object of the invention, the invention provides for a measurement of the diffusible hydrogen content in a weld seam. Depending on the determined hydrogen content, a cold cracking probability is recorded to determine the suitability of the join. In other words, the method thus comprises in particular the following method steps: First, the steel sheets are cleaned and, if necessary, dezincified to provide a clean surface. Subsequently, the steel sheets are electrolytically charged with hydrogen. For example, different charging times are used to generate different hydrogen concentrations in the material. Optionally, the charged sheets can be galvanized to preserve the hydrogen in the material, particularly if the sheets have a longer service life before the welding tests.

[0016] In the next step, the loaded steel sheets are processed using various welding processes such as laser welding, resistance spot welding (WPS), or gas metal arc welding (GMAW). For each loading time, for example, at least 10 samples are created, with the sample geometries being adapted according to the welding process used. The cold crack probabilities of the samples are determined depending on the loading time. The cold crack evaluation can be carried out according to the procedure described in DE102023002714A1. The hydrogen content in the weld metal is determined according to the DIN ISO 3690 standards or according to the method described in DE102023002911A1. For statistical validation, at least three welds are analyzed per loading time.

[0017] In addition, a cold cracking probability curve is preferably created as a function of the measured hydrogen content. This curve shows a threshold value above which the tendency to cold cracking increases sharply. Using a sigmoid plot and logistic regression, for example, the threshold value for the tendency to cold cracking is determined, with the expected cold cracking tendency for higher hydrogen contents also being calculated. This method enables a precise determination of the joining suitability of steel sheets for automotive construction, taking into account the cold cracking tendency, which depends on the hydrogen content in the weld metal.

[0018] A further aspect of the invention relates to a joining system for determining the tendency to cold cracking and / or hydrogen embrittlement of a sheet metal component during welding, comprising a clamping device for clamping a first sheet and for defined prestressing using an adjustable force. Furthermore, the joining system comprises a welding device, in particular a laser welding device, for carrying out the welding process, for example, with a complete overlap while maintaining a degassing gap between a first sheet and a second sheet. Furthermore, the joining system comprises a monitoring unit for measuring the stresses generated during the welding process and for recording the welding parameters. Finally, the joining system can comprise an image processing system for optically assessing and documenting the weld seams for cold cracking, including the option of conducting magnified visual inspections.To solve the problem, the joining system can include an evaluation unit that carries out an analysis of the critical preload force, the cold cracking severity and the susceptibility to hydrogen embrittlement, as well as a comparison of materials of the same strength class to determine the susceptibility to cold cracking.

[0019] In summary, the invention provides a method that determines the critical hydrogen content and the tendency to cold cracking at high hydrogen contents for various materials and welding processes. The idea of ​​the invention is to provide a method that makes it possible to identify the critical hydrogen content above which the tendency to cold cracking in steel sheets increases significantly, and simultaneously to predict the tendency to cold cracking at high hydrogen contents. This method contributes to improving the quality and reliability of welded steel structures by enabling control of the hydrogen concentration in weld seams and minimizing crack formation. Further advantages, features, and details of the invention will become apparent from the following description of a preferred embodiment and from the drawing.The features and combinations of features mentioned above in the description as well as the features and combinations of features mentioned below in the description of the figures and / or shown alone in the figure can be used not only in the respective combination specified, but also in other combinations or on their own, without departing from the scope of the invention.

[0020] It shows: Fig. 1 a plan view of a joining system for joining sheets together; and Fig. 2 a series of diagrams to illustrate the detection of hydrogen contents in the sheets.

[0021] In the figures, identical or functionally identical elements are provided with the same reference numerals.

[0022] Fig. 1 shows a plan view of a joining system for testing a joining of a first sheet A to a second sheet B, as well as these sheets A and B in a sectional view with the resulting weld seam C. In this figure, preferred dimensions are indicated, whereby all numbers that do not represent reference symbols are to be understood as lengths in millimeters.

[0023] In this process, the susceptibility of the material to hydrogen embrittlement is also investigated in order to detect and evaluate potential material weakening caused by diffusing hydrogen during the welding process at an early stage.

[0024] The process begins with prestressing the bottom sheet or the first sheet A using a desired force F, which can be applied, for example, by a horizontal tensile testing machine or a spring of the joining system. In the prestressed state, the top sheet or the second sheet B is welded to the first sheet A. The required degassing gap is maintained to avoid air inclusions.

[0025] The welding process is performed with a complete overlap, creating a continuous connection between the upper sheet (sheet A) and the lower sheet (sheet B). The resulting weld seam C is then tested for its susceptibility to cold cracking.

[0026] In the following evaluation, the diffusible hydrogen content in weld C is measured, and based on the determined hydrogen content, a cold cracking probability is calculated to determine the joining suitability. A cold cracking probability curve is created that represents the cold cracking probability in relation to the corresponding hydrogen content. The created cold cracking probability curve defines a threshold value above which the cold cracking tendency increases sharply. This threshold value is preferably determined using a sigmoid plot and logistic regression, with the expected cold cracking tendency for higher hydrogen contents also being calculated.

[0027] To preserve the hydrogen in the material, sheets A and B can be galvanized before welding, especially if the sheets have a longer service life before the welding tests. The welding parameters are automatically adjusted to the cold cracking tendency of the material used. Visual assessment of the cold cracking tendency is performed by an automated image processing system.

[0028] Additionally, the figure shows the indication “WD”, for “Welding Direction”, which indicates the welding direction.

[0029] Fig.Figure 2 shows a series of diagrams D1, D2, and D3 illustrating the determination of hydrogen content in various sheets or materials W1, W, and W3. Diagrams D1, D2, and D3 represent the results for materials W1, W2, and W3. The x-axis in all diagrams indicates the hydrogen content in ppm (parts per million) from 0 to 4 ppm. The y-axis indicates the cold cracking probability in percent from 0 to 100%.

[0030] The loading process of the samples / sheets / materials takes place, for example, in an aqueous electrolyte, where the samples are cathodically polarized to absorb hydrogen into the material. The loading times vary to create different hydrogen concentrations in ppm (parts per million) in the samples. The longer the loading time, the higher the hydrogen concentration in the samples. To "lock" the hydrogen into the material after loading, the samples can optionally be galvanized. This has the advantage that the samples can be aged for longer after hydrogen loading without a significant decrease in the hydrogen content.

[0031] In diagrams D1, D2 and D3, point clouds can also be seen, which represent the individual measured values ​​of the cold cracking probability as a function of the hydrogen content and form the basis for the curves created.

[0032] Diagram D1 shows a significant increase in cold cracking probability for material W1 starting at a hydrogen content of approximately 3 ppm. The curve rises to approximately 50% and then flattens out. This shows that for material W1, the critical hydrogen concentration for a significant cold cracking tendency is approximately 3 ppm.

[0033] Diagram D2 shows material W2. It shows a steep increase in the probability of cold cracking up to approximately 65% ​​at a hydrogen content of approximately 2.5 ppm. In this case, too, the curve flattens out after the increase. This indicates that material W2 is more sensitive to hydrogen content than W1, since the critical increase begins at a lower hydrogen concentration.

[0034] Diagram D3 shows material W3, where a continuous increase in cold cracking probability is observed from approximately 1 ppm to 4 ppm. In contrast to W1 and W2, there is no specific point at which the cold cracking probability increases abruptly, but rather a smooth increase across the entire range. This indicates a higher sensitivity of W3 to hydrogen in the material.

[0035] For material W1, the cold cracking tendency is approximately 55% at a crack-critical hydrogen content of 1.50 ppm.

[0036] For material W2, the cold cracking tendency is approximately 70% at a crack-critical hydrogen content of 1.36 ppm.

[0037] For material W3, the cold cracking tendency is approximately 10% at a crack-critical hydrogen content of 1.10 ppm.

[0038] This shows that the three materials W1, W2, and W3 react differently to hydrogen content. W1 only shows a significant tendency to cold cracking at higher hydrogen contents, while W2 exhibits a higher probability of cold cracking even at lower contents. W3, on the other hand, shows a steady increase in the probability of cold cracking without any abrupt increases.

[0039] In summary, a method for determining stress-induced hydrogen embrittlement of laser-welded AHSS is proposed. 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] DE102023002714A1 [0004, 0016] DE102023002911A1

[0016]

Claims

[1] Method for determining the joining suitability of a sheet metal component for vehicle construction, comprising the steps: - clamping of sheets (A, B) while maintaining a required degassing gap; - welding, in particular laser welding, the sheets (A, B) with heat energy per unit length, so that both sheets (A, B) are completely welded at the weld seams (C); - Determining the tendency of the sheets (A, B) to cold crack; and - deriving a joining suitability of the sheet metal component from the determined cold cracking tendency of at least one of the sheets (A, B), characterized by that a measurement of a diffusible hydrogen content is carried out in a weld seam and, depending on the determined hydrogen content, a cold cracking probability is recorded to determine the joining suitability. [2] Method according to claim 1, characterized bythat a cold cracking probability curve is created which represents the cold cracking probability in relation to the corresponding hydrogen content. [3] Method according to claim 2, characterized by that the created cold crack probability curve defines a limit value above which the tendency to cold crack increases sharply. [4] Method according to claim 3, characterized by that the limit value for the cold cracking tendency is determined using a sigmoid plot and a logistic regression, whereby the expected cold cracking tendency for higher hydrogen contents is also calculated. [5] Method according to one of the preceding claims, characterized by that the sheets (A, B) are galvanized before welding to preserve hydrogen in the material. [6] Method according to one of the preceding claims, characterized by that the welding parameters are automatically adapted to the cold cracking tendency of the material used. [7] Method according to one of the preceding claims, characterized by that the optical assessment of the tendency to cold cracks is carried out by an automated image processing system. [8] Joining system for operating a method according to one of claims 1 to 7.

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