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Computational fluid dynamics models with free surface tracking intended to simulate the melt pool produced by an electric arc usually model the electromagnetic force ignoring the deformation of the free surface. However, with an arc heat source, the electromagnetic force is known to be among the leading-order forces, especially at high currents. In addition, the free surface can undergo significant deformations, especially in the presence of metal transfer. In the present study, a generalization of the electromagnetic force model that accounts for the deformation of the free surface is therefore proposed. Test cases with a pulsed gas-metal arc that transfers one metal drop per pulse were investigated experimentally at three different travel speeds to provide validation data. The cases were simulated with both the proposed and the earlier model to assess the influence of the new developments. The results showed that, in the regions where both models determine the force, the discrepancy between the models’ results can reach up to an order of magnitude. Especially, the earlier model overestimates the electromagnetic force deep into the melt pool. On the other hand, it neglects it in the liquid metal that is located at an elevation above the original upper surface of the workpiece, while the proposed model showed that in this area the intensity of the electromagnetic force is the largest. These significant discrepancies result in non-negligible differences in the predicted melt pool thermal flow and geometry. Especially, the propose dextended model provides an improved prediction of the fingertip-shaped fusion boundary., CC BY 4.0

Metal additive manufacturing (AM) is used to build and repair metal parts with heat input from a high-power beam (laser or electron beam) or electric arc (gas metal, gas tungsten, or gas plasma arc) and metal deposited layer by layer from powder or wire feedstock. This multiscale and multiphysics process is difficult to directly observe. It's modeling thus contributes to gaining process understanding. It can also support process parameter selection, process control, path planning, modeling and analysis of microstructure, distortion, and residual stress formed during metal AM. The sections of this chapter address first the electron beam, laser beam, and electric arc heat sources including their interaction with metal. Next, it covers process modeling with the thermal and thermo-fluid approaches and metal provided in the form of either powder or wire feedstock. Each of these sections includes a description of the physics taking place, the existing modeling approaches, the related assumptions and framework of application, open questions, as well as examples of knowledge the models provide access to.

This study explores the possibility of tailoring the fusion zone in conduction mode laser welding using a deformable mirror for beam shaping of multi-kilowatt continuous wave laser sources. Three power density distributions were shaped and used in bead on plate welding of Ti64 plates in conduction mode at three travel speeds. The effect on melt pool free surface geometry, cross section, microstructure and hardness profiles was measured and studied. It is shown that the geometry of the melt pool can be modified using a deformable mirror. A narrower and longer melt pool or a wider, shorter and shallower one were indeed obtained forming Gaussian-elliptical power density distributions oriented along and transverse to the travel direction, respectively. The latter distribution could be a favourable option for laser beam additive manufacturing as it could improve process efficiency while reducing remelting of the previous layer. This system has also a promising potential for adaptive process control since it could change fundamentally the beam shape at a rate faster than 10 ms.

Additive manufacturing of parts on-site in space requires investigating the feasibility ofadapting to zero-gravity and near-vacuum conditions, a technology applied today on Earthat standard conditions. While a few studies have been conducted for powder bed fusion, afeasibility study remains to be explored for direct energy deposition using a laser beam anda metal wire. This is the purpose of this study, which is conducted using a modelingapproach based on computational fluid dynamics. The simulation model developedincludes melting, re-solidification, vaporization, prediction of beam energy absorptionas a function of the local surface temperature and curvature, ray tracing, tracking of freesurface deformation and metal transfer, and wire-resistive heating. The study is carried outby starting from process parameters suited for stable on-Earth metal deposition. Theseconditions were also studied experimentally to validate the simulation model, leading tosatisfactorily results. A total of three other test cases with ambient pressure lowered downto near-vacuum and/or gravitation down to zero are investigated. It is found that,compared to on-Earth conditions, in-space conditions can induce vaporization of themetal alloy that is large enough to result in a curvature of the melt pool free surface but toosmall to lead to the formation of a keyhole. The in-space conditions can also modify theforce balance at the liquid melt bridge between the wire and the melt pool, leading to smallchanges in the curvature and temperature field at the free surface of the wire tip. Among theobserved consequences are a small increase of the melt pool length and a small elevationof the bead height. More importantly, for process control, changing to in-space conditionsmight also affect the stability of the process, which could be assessed through the width ofthe liquid metal bridge. However, by using appropriate process control to maintain acontinuous liquid metal bridge, it is concluded that dire, This research work is supported by grants from the SwedishKnowledge Foundation projects AdOpt (20170315) and SAMw(20170060), which is gratefully acknowledged.

The present work concerns the electromagnetic force models in computational fluid dynamics simulations of melt pools produced with electric arcs. These are commonly applied to gas metal arcs with metal transfer, in welding and additive manufacturing. Metal drop impact on the melt pool is thus included in this study. The electromagnetic force models applied in literature use either numerical solutions of Poisson equations or one of the two analytical models developed by Kou and Sun, or Tsao and Wu. These models rely on assumptions for which the effect on the melt pool predictions remains to be understood. The present work thoroughly investigates those assumptions and their effects. It has been supported by dedicated experimental tests that did provide estimates of unknown model parameters and validation data. The obtained results show that the assumptions that fundamentally distinguish these three models change the electromagnetic force, including the relation between its components. These changes, which can also be spatially non-uniform, are large. As a result, these models lead to significantly different recirculation flow pattern, thermal convection, melt pool morphology, bead dimensions, and free surface response to the metal transfer. We conclude by proposing conditions in which each of these models is suited or questionable., CC BY-NC-ND license

A computational fluid dynamics approach is used to analyse the influence of beam shaping in fusion welding on melt thermal flow. Three beam shapes are studied at several welding travel speeds: a reference Gaussian profile and its elliptic elongations along and transverse to the welding travel direction. It is found that these beam shapes change not only the intensity and direction of the melt thermocapillary flow but also the flow pattern. For instance, and contrary to the other profiles, the beam shape elongated along the welding travel direction generates melt front vortices that assist metal pre-heating. It can result in deeper penetration, larger melt volume, and lower amount of thermal energy diffused into the heat affected zone. The simple elongation of a beam profile has thus a non-linear effect on the melt flow and in turn on the seam geometry as well as the temperature gradients in the heat affected zone.

Thermo-fluid dynamic and experimental approaches are used to investigate the influence of 20° uphill, downhill and sideway substrate orientation during metal deposition over a previously deposited bead in a V-groove. The computational fluid dynamic model with free surface deformation and metal transfer gives insight into the melt pool flow and causes of defect formation observed on the solidified beads. The experimental metallographs, high-speed images and computational results show good agreement. It is found that the deposition of a second layer on a smooth first layer cooled down to room temperature leads to large changes in melt pool flow patternat 20° substrate inclination compared to flat condition. It results in undercut and humps with the uphill orientation and undercut with the side inclination. Therefore, lower angle range is necessary for multilayer gas metal arc deposition for these two last configurations.

The effect of different shapes of laser beam power density distribution was investigated numerically with respect to the thermo-hydrodynamics of the melt pool during welding. The process addressed is conduction mode bead on plate welding of the Titanium alloy Ti-6Al-4V. A new solver based on the volume of fluid method to track the deformation of the melt free surface was developed in the OpenFOAM software. Experiments were conducted for the purpose of validating the model. In addition to the traditional cross-cut images of the weld bead, top view images of the melt pool were analysed to perform the validation along the 3-space dimensions. A good agreement between numerical predictions and experimental measurements was obtained, thus promising aconfident utilization of this simulation model when investigating the influence of beam shapes on the resulting weld seam. The effectof three different beam shapes on the melt pool velocity flow, temperature fields, and melt geometry were studied. It was found that the melt pool size was largest for an elliptical power density distribution with the major axis along the welding direction. The results also showed that the laser beam with Gaussian power density distribution resulted in the deepest penetration., Swedish Knowledge Foundation projects AdOpt (20170315) and SAMw (20170060)

A recent review pointed out that the existing models for gas tungsten arc coupling the electrode (a cathode) and the plasma are not yet complete enough. Their strength is to predict with good accuracy either the electric potential or the temperature field in the region delimited by the electrode and the workpiece. Their weakness is their poor ability to predict with good accuracy these two fields at once. However, both of these fields are important since they govern the heat flux to the workpiece through current density and temperature gradient. New developments have been made since then. They mainly concern the approaches addressing the electrode sheath (or space charge layer) that suffered from an underestimation of the arc temperature. These new developments are summarized and discussed, the modelling assumptions are examined, and important modelling issues that remain unexplored are underlined., Funders: ESAB

We report on an experimental study of the incubation effect during irradiation of stainless steel targets with bursts of femtosecond laser pulses at 1030 nm wavelength and 100 kHz repetition rate. The bursts were generated by splitting the pristine 650-fs laser pulses using an array of birefringent crystals which provided time separations between sub-pulses in the range from 1.5 ps to 24 ps. We measured the threshold fluence in Burst Mode, finding that it strongly depends on the bursts features. The comparison with Normal Pulse Mode revealed that the existing models introduced to explain the incubation effect during irradiation with trains of undivided pulses has to be adapted to describe incubation during Burst Mode processing. In fact, those models assume that the threshold fluence has a unique value for each number of impinging pulses in NPM, while in case of BM we observed different values of threshold fluence for fixed amount of sub-pulses but different pulse splitting. Therefore, the incubation factor coefficient depends on the burst features. It was found that incubation effect is higher in BM than NPM and that it increases with the number of sub-pulses and for shorter time delays within the burst. Two-Temperature-Model simulations in case of single pulses and bursts of up to 4 sub-pulses were performed to understand the experimental results. © Copyright SPIE.

A fluid dynamics approach to modelling of fusion welding in titanium alloys is proposed. The model considers the temporal and spatial evolution of liquid metal/gas interface to capture the transient physical effects during the heat source–material interaction of a fusion welding process. Melting and vaporisation have been considered through simulation of all interfacial phenomena such as surface tension, Marangoni force and recoil pressure. The evolution of the metallic (solid and liquid) and gaseous phases which are induced by the process enables the formation of the keyhole, keyhole dynamics, and the fully developed weld pool geometry. This enables the likelihood of fluid flow-induced porosity to be predicted. These features are all a function of process parameters and formulated as time-dependent phenomena. The proposed modelling framework can be utilised as a simulation tool to further develop understanding of defect formation such as weld-induced porosity for a particular fusion welding application. The modelling results are qualitatively compared with available experimental information., Funders: Rolls-Royce; Manufacturing Technology Centre, University of Birmingham

We report on an experimental investigation of the incubation effect during irradiation of stainless steel with bursts of ultrashort laser pulses. A series of birefringent crystals was used to split the pristine 650-fs pulses into bursts of up to 32 sub-pulses with time separations of 1.5 ps and 3 ps, respectively. The number of selected bursts was varied between 50 and 1600. The threshold fluence was measured in case of Burst Mode (BM) processing depending on the burst features, i.e. the number of sub-pulses and their separation time, and on the number of bursts. We found as many values of threshold fluence as the combinations of the number of bursts and of sub-pulses constituting the bursts set to give the same total number of impinging sub-pulses. However, existing incubation models developed for Normal Pulse Mode (NPM) return, for a given number of impinging pulses, a constant value of threshold fluence. Therefore, a dependence of the incubation coefficient with the burst features was hypothesized and experimentally investigated. Numerical solutions of the Two Temperature Model (TTM) in case of irradiation with single bursts of up to 4 sub-pulses have been performed to interpret the experimental results. © 2018 Optical Society of America.