Now showing 1 - 10 of 78
  • Publication
    Numerical and experimental assessment of liquid metal embrittlement in externally loaded spot welds
    ( 2024-01-30)
    Prabitz, Konstantin
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    Antretter, Thomas
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    Schubert, Holger
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    Hilpert, Benjamin
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    Gruber, Martin
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    Sierlinger, Robert
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    Ecker, Werner
    Zinc-based surface coatings are widely applied with high-strength steels in automotive industry. Some of these base materials show an increased brittle cracking risk during loading. It is necessary to examine electrogalvanized and uncoated samples of a high strength steel susceptible to liquid metal embrittlement during spot welding with applied external load. Therefore, a newly developed tensile test method with a simultaneously applied spot weld is conducted. A fully coupled 3D electrical, thermal, metallurgical and mechanical finite element model depicting the resistant spot welding process combined with the tensile test conducted is mandatory to correct geometric influences of the sample geometry and provides insights into the sample’s time dependent local loading. With increasing external loads, the morphology of the brittle cracks formed is affected more than the crack depth. The validated finite element model applies newly developed damage indicators to predict and explain the liquid metal embrittlement cracking onset and development as well as even ductile failure.
  • Publication
    KI zur Prozessüberwachung im Unterpulverschweißen
    Beim Unterpulverschweißen sind die Prozessgeräusche ein Indikator für eine gute Fügequalität. Diese Beurteilung kann i.d.R. nur von einer erfahrenen Fachkraft durchgeführt werden. Eine kürzlich entwickelte künstliche Intelligenz kann automatisch das akustische Prozesssignal anhand vortrainierter Merkmale klassifizieren und die Fügequalität anhand des Geräuschs beurteilen. Der Algorithmus, einmal richtig trainiert, kann den Prüfaufwand beim Unterpulverschweißen deutlich reduzieren.
  • Publication
    Process advantages of laser hybrid welding compared to conventional arc-based welding processes for joining thick steel structures of wind tower
    ( 2023-12-22) ;
    Üstündag, Ömer
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    Bakir, Nasim
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    The most common welding processes when joining thick-walled steels in the industry are arc-based welding processes such as GMAW or SAW. For this purpose, the sheets are joined in multi-layer technique, which can lead to productivity losses due to high welding times. The process-specific challenges in welding thick steels using multi-layer technique relate to the high heat input from the process. Therefore, alternative welding processes are being actively sought. A suitable alternative is provided by beam-based welding processes such as the laser hybrid welding processes, which are characterized by deep penetration welds and lower heat input. With implementation of the laser hybrid welding process in the heavy industry, such as the wind tower industry, economic benefits can be reached such as the increase in productivity by reducing the layer number, and the lower consumption of filler material and energy. When comparing SAW welded 25 mm thick steels in five to six layers and single-pass laser hybrid welding, the welding time can be reduced more than 80 % and the costs of filler material, flux and energy can be saved up to 90 %. However, the industrial use of the laser hybrid welding process is still limited to applications, where the material thickness does not exceed 15 mm due to some process-specific challenges such as the sagging, sensitivity to manufacturing tolerances such as gaps and misalignment, limited filler wire mixing, and deteriorated mechanical properties resulting from high cooling rates. To overcome these challenges, a contactless electromagnetic backing based on an externally applied AC magnetic field was used. Eddy currents are induced due to the oscillating magnetic field, and an upward-oriented Lorentz force is generated to counteract the droplets formed due to gravitational forces. It allows to weld up to 30 mm thick structural steels in a single-pass with a 20-kW fiber laser system. Additionally, the gap bridgeability and the misalignment of edges were increased to 2 mm when welding 20 mm thick steels. With the aid of the AC magnetic field, a vortex was formed in the weld root, which had a positive effect on the filler wire mixing.
  • Publication
    Application of Hybrid Laser Arc Welding for Construction of LNG Tanks Made of Thick Cryogenic 9% Ni Steel Plates
    Hybrid laser-arc welding (HLAW) was applied for butt welding of 14.5 mm thick plates of ferritic cryogenic steel X8Ni9 containing 9% Ni, which is used for manufacturing storage and transport facilities of liquefied natural gas (LNG). The weld seam formation and the achievable metallurgical and mechanical properties of the hybrid welds were investigated experimentally for two types of filler wire, an austenitic wire dissimilar to the base metal (BM) and an experimentally produced matching ferritic wire. Safe penetration and uniform distribution of the austenitic filler metal in the narrow hybrid weld could only be achieved in the upper, arc-dominated part of the weld. The pronounced heterogeneous distribution of the austenitic filler metal in the middle part and in the root area of the weld could not ensure sufficient notched impact toughness of the weld metal (WM). As a result, a decrease in the impact energy down to 17 ± 3 J was observed, which is below the acceptance level of ≥ 34 J for cryogenic applications. In contrast, the use of a matching ferritic filler wire resulted in satisfactory impact energy of the hybrid welds of up to 134 ± 52 J at the concerned cryogenic temperature of-196 °C. The obtained results contribute to an important and remarkable conversion in automated manufacturing of LNG facilities. In other words, the results will help to develop a new laser-based welding technology, where both quality and productivity are considered. The efficiency of the developed welding process has been demonstrated by manufacturing a prototype where a segment of the inner wall of large size LNG storage tank was constructed. In this concern, hybrid laser arc welding was conducted in both horizontal (2G) and vertical (3G) positions as a simulation to the actual onsite manufacturing. The prototype was fabricated twice where its quality was confirmed based on non-destructive and destructive examinations.
  • Publication
    The Identification of a New Liquid Metal Embrittlement (LME) Type in Resistance Spot Welding of Advanced High-Strength Steels on Reduced Flange Widths
    ( 2023-10-16)
    Yang, Keke
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    Meschut, Gerson
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    Liquid metal embrittlement (LME) cracking is a phenomenon observed during resistance spot welding (RSW) of zinc-coated advanced high-strength steels (AHSS) in automotive manufacturing. In this study, severe cracks are observed at the edge of the sheet under reduced flange widths. These cracks, traversing the AHSS sheet, culminate at the edge with a width of approximately 1.2 mm. Through combined numerical and experimental investigations, and material testing, these cracks are identified and validated as a new type of LME crack. The mechanism behind this crack formation is attributed to unique geometric conditions that, when compared to center welding, amplify radial material flow by ninefold to 0.87 mm. The resultant tangential tensile stresses approximate 760 MPa, which exceed the yield strength of the examined advanced high-strength steel (AHSS) under heightened temperature conditions, and when combined with liquid zinc, promote the formation of this new type of LME crack.
  • Publication
    Process Setup and Boundaries of Wire Electron Beam Additive Manufacturing of High-Strength Aluminum Bronze
    ( 2023-08-08)
    Raute, Maximilian Julius
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    In recent years, in addition to the commonly known wire-based processes of Directed Energy Deposition using lasers, a process variant using the electron beam has also developed to industrial market maturity. The process variant offers particular potential for processing highly conductive, reflective or oxidation-prone materials. However, for industrial usage, there is a lack of comprehensive data on performance, limitations and possible applications. The present study bridges the gap using the example of the high-strength aluminum bronze CuAl8Ni6. Multi-stage test welds are used to determine the limitations of the process and to draw conclusions about the suitability of the parameters for additive manufacturing. For this purpose, optimal ranges for energy input, possible welding speeds and the scalability of the process were investigated. Finally, additive test specimens in the form of cylinders and walls are produced, and the hardness profile, microstructure and mechanical properties are investigated. It is found that the material CuAl8Ni6 can be well processed using wire electron beam additive manufacturing. The microstructure is similar to a cast structure, the hardness profile over the height of the specimens is constant, and the tensile strength and elongation at fracture values achieved the specification of the raw material.
  • Publication
    Investigation of the LME Susceptibility of Dual Phase Steel with Different Zinc Coatings
    The application of anti-corrosion coated, high-strength steels in the automotive industry has increased in recent years. In combination with various zinc-based surface coatings, liquid metal embrittlement cracking can be observed in some of these materials. A high-quality, crack-free spotwelded joint is essential to realize the lightweight potential of the materials. In this work, the LME susceptibility of different coatings, which will be determined by the crack length and the occurrence rate, will be investigated using a welding under external load setup. The uncoated specimens did not show any LME. EG, GI and GA showed significantly less LME than ZM coatings. The latter coatings showed much larger crack lengths than the EG, GI and GA coatings. Furthermore, two mechanisms regarding the LME occurrence rate were observed: the occurrence of LME in zinc-magnesium coatings was theorized to be driven by the material properties of the coatings, whereas the occurrence of LME at EG, GI and GA samples was forced mainly by the application of the external tensile load. In the experimental setup of this work, the materials were exposed to unusually high mechanical loads (up to 80% of their yield strength) to evoke LME cracks.
  • Publication
    Adaptive Reparatur-Prozesskette
    Das additive Fertigungsverfahren Laser-Powder Directed Energy Depositon (LP-DED), kombiniert mit automatisierten Reverse-Engineering-Ansätzen, bietet die Möglichkeit, Bauteile effizient zu reparieren. Durch intelligente Algorithmen können im sogenannten Scangineering 3D-Scandaten von Bauteilen vorverarbeitet, ausgerichtet und parametrisiert werden. Die erkannten geometrischen Defekte werden zur Errechnung der Werkzeugwege für den additiven Aufbau verwendet und mittels des LP-DED-Prozesses aufgeschweißt und repariert. Dabei kommen vor allem die Vorteile der flexiblen Prozessführung, ein hoher Automatisierungsgrad und gute Reproduzierbarkeit zum Tragen.
  • Publication
    A General Analytical Solution for Two-Dimensional Columnar Crystal Growth during Laser Beam Welding of Thin Steel Sheets
    ( 2023)
    Artinov, Antoni
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    Karkhin, Victor A.
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    Meng, Xiangmeng
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    Bachmann, Marcel
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    A technique for calculating the main solidification parameters for a two-dimensional columnar crystal growth during complete penetration laser beam welding of thin steel sheets was developed. Given that the weld pool interface is described by Lamé curves (superellipses) within the horizontal plane of growth, general analytical solutions were derived for the geometry of the crystal axis and the corresponding growth rate and cross-sectional area of the crystal. A dimensionless analysis was performed to provide insights on the dependence of the solidification parameters on the shape and dimensions of the rear part of the weld pool boundary. The derived solutions were applied for the case of complete penetration laser beam keyhole welding of 2 mm thick 316L austenitic chromium-nickel steel sheets. It was shown that the reconstruction of the weld pool boundary with Lamé curves provides higher accuracy and flexibility compared to results obtained with elliptical functions. The validity of the proposed technique and the derived analytical solutions was backed up by a comparison of the obtained solutions to known analytical solutions and experimentally determined shapes and sizes of the crystals on the top surface of the sheet. The dimensions of the calculated crystal axis correlated well with the experimentally obtained results.
  • Publication
    Using ultrasonic atomization to recycle aluminium bronze chips for additive laser directed energy deposition
    ( 2023) ;
    Fasselt, Janek Maria
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    Kruse, Tobias
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    Klötzer, Christian
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    Kleba-Ehrhardt, Rafael
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    Choma, Tomasz
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    In the post-processing of large maritime components, a considerable amount of waste in the form of milling and grinding chips is produced. At the same time, additive manufacturing technologies have shown great potential in producing high-volume parts for maritime applications, allowing novel design approaches and short lead times. In this context, this study presents a sustainable approach to recycle and use aluminium bronze waste material, generated during post-processing of large cast ship propellers, as feedstock for laser-powder directed energy deposition. The recycling technology used to produce powder batches is inductive re-melting in combination with ultrasonic atomization. The derived metal powders are characterized using digital image analysis, powder flowability tests, scanning electron microscopy as well as energy dispersive X-ray spectroscopy. Compared to conventional metal powders produced by gas atomization, the recycled material shows excellent sphericity and a powder size distribution with a higher content of finer and coarser particles. Metallographic sections of deposited additively produced specimens show an increased hardness and reduced ductility, but also competitive densities and higher yield and ultimate tensile strength compared to cast material. The process chain shows high potential for the maritime sector to enable circular and sustainable manufacturing.