Now showing 1 - 10 of 11
  • Publication
    Influence of oscillating magnetic field on the keyhole stability in deep penetration laser beam welding
    ( 2021)
    Üstündag, Ö.
    ;
    Bakir, N.
    ;
    Gumenyuk, A.
    ;
    Rethmeier, M.
    The stability of the keyhole decreases for deep penetrated high-power laser beam welding. The keyhole tends to collapse with increasing laser power and e.g. keyhole induced porosity can occur. This study deals with the observation of the keyhole during high-power laser beam welding in partial penetration mode by means of a high-speed camera. A butt configuration of 25 mm thick structural steel and transparent quartz glass was used for the experiments. An oscillating magnetic field was applied perpendicular to the welding direction on the root side of the steel plate. The keyhole was highlighted with a coaxial diode laser. It was ascertained that the stability of the keyhole and the weld penetration depth were increased by applying an oscillating magnetic field with an oscillating frequency of 1.2 kHz and a magnetic flux density of 50 mT.
  • Publication
    Investigation of the gap bridgeability at high-power laser hybrid welding of plasma-cut thick mild steels with AC magnetic support
    ( 2021)
    Üstündag, Ö.
    ;
    Bakir, N.
    ;
    Gumenyuk, A.
    ;
    Rethmeier, M.
    One of the challenges of the high-power hybrid laser welding of thick steels is the sensitivity of the process of the process to manufacturing tolerances. This usually leads to a time-consuming preparation of the welding edges, such as milling. The study deals with the influence of the edge quality of milled and plasma-cut steel made of S355J2 with a wall thickness of 20 mm on the laser hybrid welded seam quality. Furthermore, the gap bridgeability and the tolerances towards edge misalignment was investigated. An AC magnet was used as backing support to prevent sagging and positioned under the workpiece, to generate an upwards directed electromagnetic pressure. The profiles of the edges and the gap on the top and root side were measured using a digital camera. Single-pass laser hybrid welds of plasma-cut edges could be welded using a laser beam power of just 13.7 kW. A gap bridgeability up to 2 mm and misalignment of edges up to 2 mm could be achieved successful. Additionally, the independence of the cutting side and the welding side was shown, so that samples were welded to the opposite side to their cutting. For evaluation of internal defects or irregularities, X-ray images were carried out. Charpy impact strength tests were performed to determine the toughness of the welds.
  • Publication
    In situ determination of the critical straining condition for solidification cracking during laser beam welding
    ( 2020)
    Bakir, N.
    ;
    Gumenyuk, A.
    ;
    Pavlov, V.
    ;
    Volvenko, S.
    ;
    Rethmeier, M.
    In recent years, laser beam welding has found wide applications in many industrial fields. Solidification cracks are one of the most frequently encountered welding defects that hinder obtaining a safe weld joint. Decades of research have shown that one of the main causes of such cracks is the strain and the strain rate. Obtaining meaningful measurements of these strains has always been a major challenge for scientists, because of the specific environment of the measurement range and the many obstacles, as well as the high temperature and the plasma plume. By applying novel metrology based on optical flow (OF) algorithm, the critical strain conditions for solidification crack formation for the stainless steel 1.4828 in the immediate vicinity of the solidification front was identified. The developed two-dimensional technique allows for obtaining full strain distribution in the hot cracking critical zone.
  • Publication
    Experimental and numerical study on the influence of the laser hybrid parameters in partial penetration welding on the solidification cracking in the weld root
    ( 2020)
    Bakir, N.
    ;
    Üstündag, Ö.
    ;
    Gumenyuk, A.
    ;
    Rethmeier, M.
    The aim of the present study is to investigate the influence of the laser hybrid welding parameters on the solidification cracks in the weld root for partial penetration welding. Welding trials were performed on thick-walled high-strength steels of grade S690QL under the same critical restraint intensity, with a variation of the welding velocity, wire feeding rate, and the focal position of the laser beam. It was ascertained that the welding velocity has a high impact on the solidification cracking phenomenon. A decrease in the welding speed leads to a reduction of the number of cracks in the weld root. The arc power has also a slight influence on the solidification cracking, while the change of the focal position of the laser beam shows also a remarkable effect. Besides, numerical simulation was performed to understand the thermomechanical behavior of the welds for different welding parameters to assist the interpretation of the experimental results.
  • Publication
    Development of a novel optical measurement technique to investigate the hot cracking susceptibility during laser beam welding
    ( 2019)
    Bakir, N.
    ;
    Pavlov, V.
    ;
    Zavjalov, S.
    ;
    Volvenko, S.
    ;
    Gumenyuk, A.
    ;
    Rethmeier, M.
    The weldability of materials is still for many years a highly contentious issue, particularly regarding the causes of the hot crack formation. Because of the process-related temperature and emissions, direct measurement for the arising strain in the close vicinity of the welding process is challenged. Therefore, the externally loaded hot cracking tests remain for decades the only way to determine the critical straining conditions for solidification cracking. In this study, a novel 2D in situ observation technique has been developed to analyze the strain evaluation during the welding process in the moment of crack formation. For the first time, the employed technique enabled the in situ measurement of the transient strain field at the surface of the workpiece directed to the laser beam in the critical range, where the solidification cracking normally occurs. Thus, the critical threshold strain values at high temperatures characterizing transition from crack-free to crack-concomitant welding process could be deduced. The influence of the global straining conditions on the direct local measured strain and strain rate for the stainless steel 316 L has been analyzed and discussed.
  • 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
    Investigation of solidification cracking susceptibility during laser beam welding using an in-situ observation technique
    ( 2018)
    Bakir, N.
    ;
    Gumenyuk, A.
    ;
    Rethmeier, M.
    In recent years, laser beam welding has found wide applications in many industrial fields. Solidification cracks are one of the most frequently encountered welding defects that hinder obtaining a safe weld joint. Decades of research have shown that one of the main causes of such cracks are the strain and the strain rate. Obtaining meaningful measurements of these strains has always been a major challenge for scientists, because of the specific environment of the measurement range and the many obstacles, as well as the high temperature and the plasma plume. In this study, a special experimental setup with a high-speed camera was employed to measure the strain during the welding process. The hot cracking susceptibility was investigated for 1.4301 stainless steel, and the critical strain required for solidification crack formation was locally and globally determined.
  • Publication
    Novel metrology to determine the critical strain conditions required for solidification cracking during laser welding of thin sheets
    ( 2018)
    Bakir, N.
    ;
    Pavlov, V.
    ;
    Zavjalov, S.
    ;
    Volvenko, S.
    ;
    Gumenyuk, A.
    ;
    Rethmeier, M.
    The weldability of materials is still for many years a highly contentious issue, particularly regarding the causes of the hot crack formation. Because of the process-related temperature and emissions, direct measurement for the arising strain in the close vicinity of the welding process is challenged. Therefore, the externally loaded hot cracking testes remain for decades the only way to determine the critical straining conditions for solidification cracking. In this study, a novel optical two-dimensional in situ observation technique has been developed to allow the strain evaluation during the welding process in the moment of crack formation. Additionally, the Controlled Tensile Weldability (CTW) test was used to generate the hot crack under different global straining conditions. To record the welding process and the moment of the solidification crack initiation a HDR-CMOS camera was used together with an 808 nm diode laser as an illumination source, so that the melt pool and the re-solidifying metal could be visualized in a single image. In order to obtain good temporal resolution, the frame rate of the camera was set to 1100 frame per second. The contrast in images obtained using this unique setup allows to apply the optical flow technique based on Lucas-Kanade (LK) algorithm to follow the pixels in each image sequence and then to calculate the displacement field. The strains were calculated based on the estimated displacements. Using this technique, the local strains under different global strain rate conditions has been determined and analysed. Moreover, the described procedure of the optical measurement allows to determine the real martial dependent values of critical strain characterizing transition to the hot cracking during laser welding processes. The experiments as well as the measurement has been performed on the stainless steel 316L (1.4404)
  • 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.