Experimental and numerical assessment of weld pool behavior and final microstructure in wire feed laser beam welding with electromagnetic stirring

Advantages such as element homogenization and grain refinement can be realized by introducing electromagnetic stirring into laser beam welding. However, the involved weld pool behavior and its direct role on determining the final microstructure have not been revealed quantitatively. In this paper, a 3D transient heat transfer and fluid flow model coupled with element transport and magnetic induction is developed for wire feed laser beam welding with electromagnetic stirring. The magnetohydrodynamics, temperature profile, velocity field, keyhole evolution and element distribution are calculated and analyzed. The model is well tested against the experimental results. It is suggested that a significant electromagnetic stirring can be produced in the weld pool by the induced Lorentz force under suitable electromagnetic parameters, and it shows important influences on the thermal fluid flow and the solidification parameter. The forward and downward flow along the longitudinal plane of the weld pool is enhanced, which can bring the additional filler wire material to the root of the weld pool. The integrated thermal and mechanical impacts of electromagnetic stirring on grain refinement which is confirmed experimentally by electron backscatter diffraction analysis are decoupled using the calculated solidification parameters and a criterion of dendrite fragmentation.DFG, 416014189, Simulation des Einflusses der elektromagnetisch unterstützten Durchmischung beim Laserstrahlschweißen dickwandiger Stahlbauteile mit Zusatzmateria

Numerical and experimental investigation of thermo-fluid flow and element transport in electromagnetic stirring enhanced wire feed laser beam welding

The introduction of electromagnetic stirring to laser beam welding can bring several beneficial effects e.g. element homogenization and grain refinement. However, the underlying physics has not been fully explored due to the absence of quantitative data of heat and mass transfer in the molten pool. In this paper, the influence of electromagnetic stirring on the thermo-fluid flow and element transport in the wire feed laser beam welding is studied numerically and experimentally. A three-dimensional transient heat transfer and fluid flow model coupled with dynamic keyhole, magnetic induction and element transport is developed for the first time. The results suggest that the Lorentz force produced by an oscillating magnetic field and its induced eddy current shows an important influence on the thermo-fluid flow and the keyhole stability. The melt flow velocity is increased by the electromagnetic stirring at the rear and lower regions of molten pool. The keyhole collapses more frequently at the upper part. The additional elements from the filler wire are significantly homogenized because of the enhanced forward and downward flow. The model is well verified by fusion line shape, high-speed images of molten pool and measured element distribution. This work provides a deeper understanding of the transport phenomena in the laser beam welding with magnetic field.DFG, 416014189, Simulation des Einflusses der elektromagnetisch unterstützten Durchmischung beim Laserstrahlschweißen dickwandiger Stahlbauteile mit Zusatzmateria

Study on the transition behavior of the bulging effect during deep penetration laser beam welding

The present work is devoted to the study of the transition behavior of the recently confirmed widening of the weld pool, known as the bulging effect, during high-power deep penetration laser beam welding of thick unalloyed steel sheets. A three-dimensional transient multi-physics numerical model is developed, allowing for the prediction of the bulge formation and the study of its temporal behavior. The model is generalized to account automatically for the transition from partial to complete penetration. Several experimental measurements and observations, such as drilling period, weld pool length, temperature, efficiency, and metallographic cross-sections are used to verify the model and assure the plausibility of the numerical results. The analysis of the calculated temperature and velocity distributions, as well as the evolution of the keyhole geometry, shows that the formation of a bulging region strongly depends on the penetration depth of the weld. Based on the numerical results, the bulge is found to occur transiently, having its transition from a slight bulge to a fully developed bulging between penetration depths of 6 mm and 9 mm, respectively.DFG, 411393804, Experimentelle und numerische Untersuchung der Entstehungsmechanismen des Bulgings und dessen Einfluss auf die Bildung von Mittelrippendefekten beim Hochleistungslaserstrahlschweißen niedriglegierter Stähle hoher BlechdickeDFG, 416014189, Simulation des Einflusses der elektromagnetisch unterstützten Durchmischung beim Laserstrahlschweißen dickwandiger Stahlbauteile mit Zusatzmateria

The bulging effect and its relevance in high power laser beam welding

The present work deals with the recently confirmed widening of the weld pool interface, known as a bulging effect, and its relevance in high power laser beam welding. A combined experimental and numerical approach is utilized to study the influence of the bulge on the hot cracking formation and the transport of alloying elements in the molten pool. A technique using a quartz glass, a direct-diode laser illumination, a high-speed camera, and an infrared camera is applied to visualize the weld pool geometry in the longitudinal section. The study examines the relevance of the bulging effect on both, partial and complete penetration, as well as for different sheet thicknesses ranging from 8 mm to 25 mm. The numerical analysis shows that the formation of a bulge region is highly dependent on the penetration depth and occurs more frequently during partial penetration above 6 mm and complete penetration above 8 mm penetration depth, respectively. The location of the bulge correlates strongly with the cracking location. The obtained experimental and numerical results reveal that the bulging effect increases the hot cracking susceptibility and limits the transfer of alloying elements from the top of the weld pool to the weld root.DFG, 411393804, Experimentelle und numerische Untersuchung der Entstehungsmechanismen des Bulgings und dessen Einfluss auf die Bildung von Mittelrippendefekten beim Hochleistungslaserstrahlschweißen niedriglegierter Stähle hoher BlechdickeDFG, 416014189, Simulation des Einflusses der elektromagnetisch unterstützten Durchmischung beim Laserstrahlschweißen dickwandiger Stahlbauteile mit Zusatzmateria

The influence of magnetic field orientation on metal mixing in electromagnetic stirring enhanced wire feed laser beam welding

The application of the electromagnetic stirring from an oscillating magnetic field can improve the metal mixing in wire feed laser beam welding. However, the extra parameters introduced in this technique make the selection of an optimal combination of process parameters more difficult. In the current study, besides the commonly concerned magnetic flux density and frequency, the influence of the magnetic field orientation (magnetic field angle) on the transport of filler metal is studied numerically and experimentally. Ex-situ X-ray fluorescence spectrometer measurements are used to map the metal mixing in the final weld. A three-dimensional transient multi-physical model is developed to reveal the deeper physical essence, considering the coupling between heat transfer, fluid flow, keyhole dynamics, element transport and magnetohydrodynamics. The spatial distribution of the laser energy on the keyhole wall is calculated by a ray tracing algorithm. The results show that the magnetic field with smaller angle with respect to the transverse direction provides better penetration capacity, and its stirring effect can reach the lower part of the molten pool. Therefore, the smaller magnetic field angle produces better metal mixing. A constant downward flow is formed at the lower part of the molten pool when magnetic field of 10° angle is applied, which brings the filler metal to the root region. As the magnetic field angle increases to 40°, the beneficial downward flow changes into a constant upward flow, resulting in a concentration of the filler metal in the upper region. This study provides further insight of the underlying physics in the electromagnetically enhanced laser beam welding, which may guide the optimization of parameters to achieve property homogeneity or to avoid potential defects.DFG, 416014189, Simulation des Einflusses der elektromagnetisch unterstützten Durchmischung beim Laserstrahlschweißen dickwandiger Stahlbauteile mit Zusatzmateria

Theoretical study of influence of electromagnetic stirring on transport phenomena in wire feed laser beam welding

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in X. Meng et al., Journal of Laser Applications 32, 022026 (2020) and may be found at https://doi.org/10.2351/7.0000069.The additional element from the filler wire in the laser beam welding is usually distributed inhomogeneously in the final weld due to the high solidification rate of weld pool. It has been found that the electromagnetic stirring produced by an external oscillating magnetic field can enhance the material mixing in the weld pool to achieve a more uniform element distribution. However, the magnetic field has a highly nonlinear and multicoupled interaction with the weld pool behavior, which makes the quantitative explanation of the physical mechanism difficult. In this study, the effect of electromagnetic stirring on the transport phenomena in the wire feed laser beam welding is investigated by a numerical modeling. A 3D transient multiphysical model considering the magnetohydrodynamics, heat transfer, fluid flow, keyhole dynamics, and element transport is developed. The multiple reflections and the Fresnel absorption of the laser on the keyhole wall are calculated using the ray tracing method. The numerical results show that a Lorentz force produced by the oscillating magnetic field and its induced eddy current gives significant influence on the transport phenomena in the molten pool. The forward and downward flow is enhanced by the electromagnetic stirring, which homogenizes the distribution of the additional elements from a nickel-based filler wire in a steel weld pool. The numerical results show a good agreement with the high-speed images of the molten pool, the fusion line from the optical micrograph, and the element distribution from the energy dispersive x-ray spectroscopy. This work provides a physical base for the electromagnetic-controlled laser beam welding and some guidance for the selection of electromagnetic parameters.DFG, 416014189, Simulation des Einflusses der elektromagnetisch unterstützten Durchmischung beim Laserstrahlschweißen dickwandiger Stahlbauteile mit Zusatzmateria

Numerical Analysis of the Partial Penetration High Power Laser Beam Welding of Thick Sheets at High Process Speeds

The present work is devoted to the numerical analysis of the high-power laser beam welding of thick sheets at different welding speeds. A three-dimensional transient multi-physics numerical model is developed, allowing for the prediction of the keyhole geometry and the final penetration depth. Two ray tracing algorithms are implemented and compared, namely a standard ray tracing approach and an approach using a virtual mesh refinement for a more accurate calculation of the reflection point. Both algorithms are found to provide sufficient accuracy for the prediction of the keyhole depth during laser beam welding with process speeds of up to 1.5mmin−1. However, with the standard algorithm, the penetration depth is underestimated by the model for a process speed of 2.5mmin−1 due to a trapping effect of the laser energy in the top region. In contrast, the virtually refined ray tracing approach results in high accuracy results for process speeds of both 1.5mmin−1 and 2.5mmin−1. A detailed study on the trapping effect is provided, accompanied by a benchmark including a predefined keyhole geometry with typical characteristics for the high-power laser beam welding of thick plates at high process speed, such as deep keyhole, inclined front keyhole wall, and a hump.DFG, 411393804, Experimentelle und numerische Untersuchung der Entstehungsmechanismen des Bulgings und dessen Einfluss auf die Bildung von Mittelrippendefekten beim Hochleistungslaserstrahlschweißen niedriglegierter Stähle hoher BlechdickeDFG, 416014189, Simulation des Einflusses der elektromagnetisch unterstützten Durchmischung beim Laserstrahlschweißen dickwandiger Stahlbauteile mit Zusatzmateria

Stereoselective property of 20(S)-protopanaxadiol ocotillol type epimers affects its absorption and also the inhibition of P-glycoprotein.

Stereoselectivity has been proved to be tightly related to drug action including pharmacodynamics and pharmacokinetics. (20S,24R)-epoxy-dammarane-3,12,25-triol (24R-epimer) and (20S,24S)-epoxy-dammarane-3,12,25-triol (24S-epimer), a pair of 20(S)-protopanaxadiol (PPD) ocotillol type epimers, were the main metabolites of PPD. Previous studies have shown that 24R-epimer and 24S-epimer had stereoselectivity in pharmacological action and pharmacokinetics. In the present study, the aim was to further study the pharmacokinetic characteristics of both epimers, investigate their absorption mechanism and analyze the selectivity effects of ocotillol type side chain and C24 stereo-configuration on P-glycoprotein (P-gp) in vivo and in vitro. Results showed that the absolute bioavailability of 24R-epimer was about 14-fold higher than that of 24S-epimer, and a linear kinetic characteristic was acquired in doses of 5-20 mg/kg for both epimers after oral administration. Furthermore, the apparent permeability coefficients of 24R-epimer were 5-7 folds higher than that of 24S-epimer having lower efflux ratios in Caco-2 cell models. Moreover, both 24R-epimer and 24S-epimer had similar inhibitory effects on P-gp by increasing cellular retention of rhodamine 123 in Caco-2 cells and decreasing efflux of digoxin across Caco-2 cell monolayers. In situ in vivo experiments showed that the inhibition of 24R-epimer on P-gp was stronger than that of 24S-epimer by single-pass intestinal perfusion of rhodamine 123 in rats. Western blot analyses demonstrated that both epimers had no action on P-gp expression in Caco-2 cells. In conclusion, with respect to the stereoselectivity, C24 S-configuration of the ocotillol type epimers processed a poor transmembrane permeability and could be distinguished by P-gp. Sharing a dammarane skeleton, both 24R-epimer and 24S-epimer were potent inhibitors of P-gp. This study provides a new case of stereoselective pharmacokinetics of chiral compounds which contributes to know the chiral characteristics of P-gp and structure-action relationship of PPD type and ocotillol type ginsenosides as a P-gp inhibitor

Lamé curve approximation for the assessment of the 3D temperature distribution in keyhole mode welding processes

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in A. Artinov et al., Journal of Laser Applications 32, 022042 (2020) and may be found at https://doi.org/10.2351/7.0000076.A novel approach for the reconstruction of an equivalent volumetric heat source from a known weld pool shape is proposed. It is based on previously obtained weld pool geometries from a steady-state thermo-fluid dynamics simulation. Hereby, the weld pool dimensions are obtained under consideration of the most crucial physical phenomena, such as phase transformations, thermo-capillary convection, natural convection, and temperature-dependent material properties. The algorithm provides a time and calibration efficient way for the reproduction of the weld pool shape by local Lamé curves. By adjusting their parameters, the identification of the finite elements located within the weld pool is enabled. The heat input due to the equivalent heat source is assured by replacing the detected nodes' temperature by the melting temperature. The model offers variable parameters making it flexible and adaptable for a wide range of workpiece thicknesses and materials and allows for the investigation of transient thermal effects, e.g., the cooling stage of the workpiece. The calculation times remain acceptably short especially when compared to a fully coupled process simulation. The computational results are in good agreement with performed complete-penetration laser beam welding experiments.DFG, 411393804, Experimentelle und numerische Untersuchung der Entstehungsmechanismen des Bulgings und dessen Einfluss auf die Bildung von Mittelrippendefekten beim Hochleistungslaserstrahlschweißen niedriglegierter Stähle hoher BlechdickeBMWi, 19582N, Investigation of the influence of the restraint conditions on hot cracking in laser and laser-hybrid welding of thick structure steel

Effects of 24R-epimer and 24S-epimer on P-gp expression levels in Caco-2 cells.

<p>Cells were exposed to 24R-epimer or 24S-epimer at concentrations of 1 µM (A) and 10 µM (B), and 1% of DMSO as a solvent control for 48 and 72 h. The bar graphs show the P-gp protein band intensity corrected by that of β-actin.</p