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 joining suitability of a sheet metal component for vehicle construction, comprising the steps of: - clamping sheets (A, B) while maintaining a required degassing gap; - welding the sheets (A, B) in complete overlap with line energy, so that both sheets (A, B) are completely welded through; - determining a property of at least one of the sheets (A, B); and - deriving a joining suitability of the sheet metal component from the determined property of at least one of the sheets (A, B). According to the invention, at least one slow tensile test is performed to determine strength values ​​as a property of the at least one sheet (A, B), and the strength values ​​determined thereby are compared. Furthermore, the invention 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 of the sheets, e.g. in complete overlap with heat energy, so that both sheets are completely welded; - Determining a property of at least one of the sheets; and - Deriving a joining suitability of the sheet metal component from the determined property 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 together; the welding method will be discussed below. The resulting weld joins the two sheets together after the welding process is completed by continuously combining the molten material from the first sheet with the weld metal and the molten material from the second sheet.

[0010] The weld can then be tested for cold cracking and / or delayed hydrogen embrittlement, for example, by examining the resulting sample at specified intervals. In the initial step, observation 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 selected 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.

[0011] 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.

[0012] This method is used to test cold crack susceptibility during 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.

[0013] 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.

[0014] To achieve the object of the invention, the invention provides that at least one slow tensile test is conducted to determine strength values ​​as a property of at least one sheet, and a comparison of the strength values ​​determined thereby is performed. A slow tensile test is a type of test in which a sheet is stretched very slowly under controlled load in order to analyze its strength and fracture behavior. The slow increase in load allows time- and strain-dependent effects, such as material embrittlement or corrosion, to be recorded and analyzed.

[0015] It is known from the state of the art that base material information does not provide any information about the tendency to hydrogen embrittlement. To date, only quantitative testing methods exist for evaluating the influence of post-welding service life and the pre-welding surface condition on the delayed softening of laser welds under load. Hydrogen diffusion processes that weaken the material depend not only on time and load, but also on the corrosive environment. To date, there is no comparable testing option for analyzing the influence of external hydrogen sources on hydrogen embrittlement and softening of laser welds under load.

[0016] The known specimen geometries and test speeds are used, with the key difference that external loading of the weld seam does not occur in a corrosive environment. The goal is therefore to develop test specimens, fixtures, and / or the method proposed here to determine the delayed softening or hydrogen embrittlement tendency of a steel under pulsating load and in a corrosive environment.

[0017] To solve this problem, it is planned to use specially developed, externally loaded specimens with a stitched weld and end crater. These specimens undergo the test procedure and clearly and reproducibly quantify the decrease in the strength properties (load-bearing capacity and elongation) of a weld under increasing load in a corrosive environment. This reveals the hydrogen embrittlement tendency of a steel.

[0018] A further aspect of the invention relates to a joining system for operating a method according to the first aspect. The aim is thus to determine the tendency to cold cracking and / or hydrogen embrittlement of a sheet metal component during welding. The joining system comprises 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, a resistance spot welding device, and / or a metal arc welding device, for carrying out the welding process with 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 include an image processing system for optical assessment and documentation of weld seams for cold cracking, including the ability to conduct magnified visual inspections. Furthermore, the joining system can include an evaluation unit that performs an analysis of the critical preload force, cold crack severity, and susceptibility to hydrogen embrittlement, as well as enabling a comparison of materials of the same strength class to determine cold cracking susceptibility.

[0019] In summary, the invention proposes a method for determining stress-induced hydrogen embrittlement of laser-welded AHSS steels (Advanced High-Strength Steels), which method at least partially improves process stability and product quality when welding high-strength steels.

[0020] Further advantages, features, and details of the invention will become apparent from the following description of a preferred embodiment and from the drawings. 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 combinations specified, but also in other combinations or on their own, without departing from the scope of the invention.

[0021] It shows: Fig. 1 Top view of a joining system for joining sheets of metal together.

[0022] In the figure, identical or functionally identical elements are provided with the same reference numerals.

[0023] Fig.Figure 1 shows a plan view of a joining system in which a joining of a first sheet A to a second sheet B is being tested. Preferred dimensions are indicated in this figure, with all numbers other than reference symbols being understood as lengths in millimeters.

[0024] Three different welding processes are presented: laser welding (LS), resistance spot welding (WPS), and gas metal arc welding (GMAW). These processes can be distinguished by the respective weld seams C1, C2, and C3, allowing the representation of the different types of joints between sheets A and B.

[0025] Laser welding (LS) uses a highly focused laser beam to precisely melt the metal sheets, creating a deep, penetrating C1 weld seam suitable for fine and precise joints. Resistance spot welding (WPS), on the other hand, uses electrical current and pressure to join metal sheets A and B at defined points, or weld seams (C2). This creates a spot weld at the contact points. In gas-shielded metal arc welding (GMAW), an arc is generated between a consumable electrode and the workpiece. A shielding gas surrounds the weld to prevent oxidation, creating a strong and particularly robust C3 weld seam.

[0026] A process sequence tailored to LS laser beam welding involves clamping and welding sheets A and B in a complete overlap using heat energy per unit length, so that both sheets A and B are fully welded through, while maintaining a degassing gap. Slow tensile tests are performed at a speed of v < 0.001 mm / s. The welded specimens are tested in air and in a corrosive medium (e.g., electrolyte, 5% NaCl). The subsequent comparison of the reduced strength values ​​provides information about the weld softening due to slowly increasing load in a corrosive environment.

[0027] A process sequence tailored to resistance spot welding (WPS) and / or gas metal arc welding (GMAW) also requires that sheets A and B are clamped and welded in an overlapping manner using heat energy per unit length, if necessary, maintaining a degassing gap. Slow tensile tests are performed at a speed of v < 0.0001 mm / s. Here, too, the specimens are tested both in air and in a corrosive medium (e.g., electrolyte, 5% NaCl). The subsequent comparison of the reduced strength values ​​demonstrates the weld softening due to slowly increasing load in a corrosive environment.

[0028] In summary, the invention proposes a method for determining the time-delayed hydrogen embrittlement of joints in corrosive media. 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]

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 the sheets (A, B) with heat energy so that both sheets (A, B) are completely welded; - determining a property of at least one of the sheets (A, B); and - deriving a joining suitability of the sheet metal component from the determined property of at least one of the sheets (A, B), characterized by that at least one slow tensile test is carried out to determine strength values as the property of the at least one sheet (A, B) and a comparison of strength values determined thereby is carried out. [2] Method according to claim 1, characterized by that a slow tensile test is carried out on sheets (A, B) welded in air. [3] Method according to claim 1 or 2, characterized bythat a slow tensile test is carried out on sheets (A, B) welded in a corrosive medium. [4] Method according to one of the preceding claims, characterized by that the welding is carried out as laser beam welding. [5] Method according to one of the preceding claims, characterized by that the welding is carried out as resistance spot welding. [6] Method according to one of the preceding claims, characterized by that the welding is carried out as metal arc welding. [7] Joining system for operating a method according to one of claims 1 to 6.

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