Fraunhofer-Institut für Produktionsanlagen und Konstruktionstechnik IPK

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  • Publication
    On the search for the origin of the bulge effect in high power laser beam welding
    ( 2019)
    Artinov, A.
    ;
    Bakir, N.
    ;
    Bachmann, M.
    ;
    Gumenyuk, A.
    ;
    Na, S.-J.
    ;
    Rethmeier, M.
    The shape of the weld pool in laser beam welding plays a major role in understanding the dynamics of the melt and its solidification behavior. The aim of the present work was its experimental and numerical investigation. To visualize the geometry of the melt pool in the longitudinal section, a butt joint configuration of 15 mm thick structural steel and transparent quartz glass was used. The weld pool shape was recorded by means of a high-speed video camera and two thermal imaging cameras, a mid-wavelength infrared camera and a newly developed infrared camera working in the spectral range of 500 to 540 nm, making it perfectly suited for temperature measurements of molten materials. The observations show that the dimensions of the weld pool vary depending on the depth. The regions close to the surface form a teardrop-shaped weld pool. A bulge region and its temporal evolution were observed approximately in the middle of the depth of the weld pool. Additionally, a transient numerical simulation was performed until reaching a steady state to obtain the weld pool shape and to understand the formation mechanism of the observed bulging phenomena. A fixed keyhole with an experimentally obtained shape was used to represent the full-penetration laser beam welding process. The model considers the local temperature field, the effects of phase transition, thermocapillary convection, natural convection, and temperature-dependent material properties up to evaporation temperature. It was found that the Marangoni convection and the movement of the laser heat source are the dominant factors for the formation of the bulge region. A good correlation between the numerically calculated and the experimentally observed weld bead shapes and the time-temperature curves on the upper and bottom surface was found.
  • Publication
    Numerical simulation on the origin of solidification cracking in laser welded thick-walled structures
    ( 2018)
    Bakir, N.
    ;
    Artinov, A.
    ;
    Gumenyuk, A.
    ;
    Bachmann, M.
    ;
    Rethmeier, M.
    One of the main factors affecting the use of lasers in the industry for welding thick structures is the process accompanying solidification cracks. These cracks mostly occurring along the welding direction in the welding center, and strongly affect the safety of the welded components. In the present study, to obtain a better understanding of the relation between the weld pool geometry, the stress distribution and the solidification cracking, a three-dimensional computational fluid dynamic (CFD) model was combined with a thermo-mechanical model. The CFD model was employed to analyze the flow of the molten metal in the weld pool during the laser beam welding process. The weld pool geometry estimated from the CFD model was used as a heat source in the thermal model to calculate the temperature field and the stress development and distributions. The CFD results showed a bulging region in the middle depth of the weld and two narrowing areas separating the bulging region from the top and bottom surface. The thermo-mechanical simulations showed a concentration of tension stresses, transversally and vertically, directly after the solidification during cooling in the region of the solidification cracking.
  • Publication
    Weld pool shape observation in high power laser beam welding
    ( 2018)
    Artinov, A.
    ;
    Bakir, N.
    ;
    Bachmann, M.
    ;
    Gumenyuk, A.
    ;
    Rethmeier, M.
    The geometry of the melt pool in laser beam welding plays a major role to understand the dynamics of the melt and its solidification behavior. In this study, a butt configuration of 15 mm thick structural steel and transparent quartz glass was used to observe the weld pool geometry by means of high-speed camera and an infrared camera recording. The observations show that the dimensions of the weld pool vary depending on the depth. The areas close to the weld pool surface take a teardrop-shape. A bulge-region and its temporal evolution were observed approximately in the middle of the depth of the weld pool. Additionally, a 3D transient thermal-fluid numerical simulation was performed to obtain the weld pool shape and to understand the formation mechanism of the observed bulging effect. The model takes into account the local temperature field, the effects of phase transition, thermo-capillary convection, natural convection and temperature-dependent material properties up to evaporation temperature. The numerical results showed good accordance and were furthermore used to improve the understanding of the experimentally observed bulging effect.