Fraunhofer-Institut für Produktionsanlagen und Konstruktionstechnik IPK

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  • Publication
    Investigation of liquid metal embrittlement avoidance strategies for dual phase steels via electro-thermomechanical finite element simulation
    ( 2022-06)
    Biegler, Max
    ;
    Böhne, Christoph
    ;
    Seitz, Georg
    ;
    Meschut, Gerson
    ;
    Rethmeier, Michael
    Modern advanced high-strength steel (AHSS) sheets used in automotive body construction are mostly zinc coated for corrosion resistance. The presence of zinc can cause cracking in steels due to liquid metal embrittlement (LME) during resistance spot welding (RSW). In combination with factors such as tensile strains, liquid zinc can lead to the formation of brittle, intergranular cracks in the weld and heat affected zone. While practical investigations to mitigate LME occurrence exist, the reason why a certain parameter might cause or prevent LME is often unknown. Numerical resistance spot welding simulation can visualize the underlying stresses, strains and temperatures during the welding process and investigate experimentally unmeasurable phenomena. In this work, a 3-dimensional electro-thermomechanical finite element approach is used to assess and investigate the critical parameters leading to LME occurrence. Experimentally observed crack sizes are correlated with the corresponding local strain rates and temperature exposure durations in the simulation. With this data, a map of LME occurrence over driving influence factors is drafted and discussed for effectiveness.
  • Publication
    Avoidance of End Crater Imperfections at High-Power Laser Beam Welding of Closed Circumferential Welds
    ( 2019)
    Gook, Sergej
    ;
    Üstündag, Ömer
    ;
    Gumenyuk, Andrey
    ;
    Rethmeier, Michael
    The present work deals with the development of a strategy for the prevention of end crater defects in high-power laser welding of thick-walled circumferential welds. A series of experiments were performed to understand the influence of the welding parameters on the formation of the imperfections such as pores, cracks, excessive root-side drop-through and shrinkage cavities in the overlap area. An abrupt switch-out of the laser power while closing the circumferential weld leads to a formation of a hole which passes through the whole welded material thickness. A laser power ramp causes solidification cracks which are initiated on the transition from full-penetration mode to partial penetration. Strategies with a reduction of the welding speed shows a creation of inadmissible root sagging. Defocusing the laser beam led to promising results in terms of avoiding end crater imperfections. Cracks and pores could be effectively avoided by using defocusing techniques. A strategy for avoiding of end crater defects was tested on flat specimens of steel grade S355J2 with a wall thickness of 10 mm and then transferred on the 9.5 mm thick pipe sections made of high-strength steel X100Q.
  • Publication
    Hybrid laser arc welding of 25 mm thick materials using electromagnetic weld pool support
    ( 2018)
    Üstündag, Ömer
    ;
    Avilov, Vjaceslav
    ;
    Gumenyuk, Andrey
    ;
    Rethmeier, Michael
    In addition to the many advantages of deep penetration, increased welding speed and a low sensitivity to manufacturing tolerances such as gap and edge offset, the hybrid laser arc welding process is used increasingly in industrial applications such as shipbuilding or pipeline manufacturing. Nonetheless, thick-walled sheets with a wall thickness of 20 mm or more are still multi-pass welded using the arc welding process, due to increased process instability by increasing laser power. Welding at reduced speed, especially in a flat position, leads to an irregular formation of the root part such as dropping. The hydrostatic pressure exceeds the surface tension, which decreases with increasing seam width. In order to prevent gravity drop-outs, the use of a melt pool support is necessary. Usual weld pool supports such as ceramic or powder supports require time-consuming mechanical detachment. The electromagnetic weld pool support system, which is described in this study, operates without contact and based on generating Lorentz forces in the weld pool. An externally applied oscillating magnetic field induces eddy currents and generates an upward directed Lorentz force, which counteracts the hydrostatic pressure. This allows single-pass welds up to 25 mm by hybrid laser arc welding process with a 20-kW fibre laser. Moreover, it is favoured by the diminished welding speed the cooling rate which leads to an improvement of the mechanical-technological properties of the seams - the lower formation of martensite in the microstructure enables better Charpy impact toughness. The electromagnetic weld pool support extends the limitation of the laser hybrid welding process in the thick sheet area. By adapting the electromagnetic weld pool support to the laser and laser hybrid welding process, the application potential of these technologies for industrial implementation can be drastically increased.