Internal flow behaviour during melt-pool-based metal manufacturing remains unclear and hinders progression to process optimisation. In this contribution, we present direct time-resolved imaging of melt pool flow dynamics from a high-energy synchrotron radiation experiment. We track internal flow streams during arc welding of steel and measure instantaneous flow velocities ranging from 0.1 m s⁻¹ to 0.5 m s⁻¹. When the temperature-dependent surface tension coefficient is negative, bulk turbulence is the main flow mechanism and the critical velocity for surface turbulence is below the limits identified in previous theoretical studies. When the alloy exhibits a positive temperature-dependent surface tension coefficient, surface turbulence occurs and derisory oxides can be entrapped within the subsequent solid as result of higher flow velocities. The widely used arc welding and the emerging arc additive manufacturing routes can be optimised by controlling internal melt flow through adjusting surface active elements.

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An experimental and numerical study has been made of oil-water core-annular flow in a horizontal pipe with special attention for the turbulence in the water. An experimental set-up was built to be able to compare numerical predictions with detailed experimental results. The oil density was considerably smaller than the water density. Full 3D and time-dependent numerical simulations of some of the experiments were done. Only when a turbulence model is applied the agreement with the experiments is reasonably good.@en

Flows in low Prandtl number liquid pools are relevant for various technical applications and have so far only been investigated for the case of pure fluids, i.e., with a constant, negative surface tension temperature coefficient ∂γ/γT. Real-world fluids containing surfactants have a temperature dependent ∂γ/γT > 0, which may change sign to ∂γ/∂T <0 at a critical temperature Tc. Where thermocapillary forces are the main driving force, this can have a tremendous effect on the resulting flow patterns and the associated heat transfer. Here we investigate the stability of such flows for five Marangoni numbers in the range of 2.1 × 10⁶ ≤ Ma ≤ 3.4 × 10⁷ using dynamic large eddy simulations, which we validate against a high resolution direct numerical simulation. We find that the five cases span all flow regimes, i.e., stable laminar flow at Ma ≤ 2.1 × 10⁶, transitional flow with rotational instabilities at Ma = 2.8 × 10⁶ and Ma = 4.6 × 10⁶, and turbulent flow at Ma = 1.8 × 10⁷ and Ma = 3.4 × 10⁷.

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Experimental observations of high-energy surface melting processes, such as laser welding, have revealed unsteady, often violent, motion of the free surface of the melt pool. Surprisingly, no similar observations have been reported in numerical simulation studies of such flows. Moreover, the published simulation results fail to predict the post-solidification pool shape without adapting non-physical values for input parameters, suggesting the neglect of significant physics in the models employed. The experimentally observed violent flow surface instabilities, scaling analyses for the occurrence of turbulence in Marangoni driven flows, and the fact that in simulations transport coefficients generally have to be increased by an order of magnitude to match experimentally observed pool shapes, suggest the common assumption of laminar flow in the pool may not hold, and that the flow is actually turbulent. Here, we use direct numerical simulations (DNS) to investigate the role of turbulence in laser melting of a steel alloy with surface active elements. Our results reveal the presence of two competing vortices driven by thermocapillary forces towards a local surface tension maximum. The jet away from this location at the free surface, separating the two vortices, is found to be unstable and highly oscillatory, indeed leading to turbulence-like flow in the pool. The resulting additional heat transport, however, is insufficient to account for the observed differences in pool shapes between experiment and simulations.

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Through an integrated macroscale/mesoscale computational model, we investigate the developing shape and grain morphology during the melting and solidification of a weld. In addition to macroscale surface tension driven fluid flow and heat transfer, we predict the solidification progression using a mesoscale model accounting for realistic solidification kinetics, rather than quasi-equilibrium thermodynamics. The tight coupling between the macroscale and the mesoscale distinguishes our results from previously published studies. The inclusion of Marangoni driven fluid flow and heat transfer, both during heating and cooling, was found to be crucial for accurately predicting both weld pool shape and grain morphology. However, if only the shape of the weld pool is of interest, a thermodynamic quasi-equilibrium solidification model, neglecting solidification kinetics, was found to suffice when including fluid flow and heat transfer. We demonstrate that the addition of a sufficient concentration of approximately 1 pin diameter TiN grain refining particles effectively triggers a favorable transition from columnar dendritic to equiaxed grains, as it allows for the latter to heterogeneously nucleate in the undercooled melt ahead of the columnar dendritic front. This transition from columnar to equiaxed growth is achievable for widely differing weld conditions, and its precise nature is relatively insensitive to the concentration of particles and to inaccurately known model parameters. (C) 2015 Elsevier Ltd. All rights reserved.@en

Abstract In the literature on numerical simulations of Marangoni driven hydrodynamics in laser spot weld pools, it has been proven impossible to obtain good agreement between simulated and experimentally observed weld shapes without artificially enhancing the thermal conductivity and the dynamic viscosity of the liquid steel by one to two orders of magnitude. This has mostly been ascribed to flow instabilities that are not properly accounted for in the simulations. However, whereas experimental weld shapes are obtained post solidification, the cooling and solidification stage is generally neglected in reported simulations. In the present work, we report a detailed study on the role of the artificial diffusivity enhancement factors in weld pool simulations, and we extend the simulations into the cooling and solidification stage. We show that during the cooling stage, flow reversal may occur in the weld pool, which enhances the downward heat and momentum transfer. This leads to a deeper weld pool that agrees better with experimental results. By including the cooling and solidification stage into the simulations of weld pool hydrodynamics, an improved agreement with experimentally observed weld shapes can be obtained with a reduced necessity to artificially enhance the thermal conductivity and the dynamic viscosity@en

Various physical interfacial phenomena occur during the process of welding and influence the final properties of welded structures. As the features of such interfaces depend on physics that resolve at different spatial scales, a multiscale and multiphysics numerical modeling approach is necessary. In a collaborative research project Modeling of Interface Evolution in Advanced Welding, a novel strategy of model linking is employed in a multiscale, multiphysics computational framework for fusion welding. We only directly link numerical models that are on neighboring spatial scales instead of trying to link all submodels directly together through all available spatial scales. This strategy ensures that the numerical models assist one another via smooth data transfer, avoiding the huge difficulty raised by forcing models to attempt communication over many spatial scales. Experimental activities contribute to the modeling work by providing valuable input parameters and validation data. Representative examples of the results of modeling, linking and characterization are presented.@en