Melt Pool Behavior Research Articles

Simultaneous high-speed ultrasound and synchrotron x-ray imaging for monitoring melt pool dynamics in metallic additive manufacturing

Fundamental to metallic additive manufacturing (AM) is the laser-powder-substrate interaction that leads to localized melting of metallic powder to a previously solidified substrate. The formation of the melt pool and the subsequent solidification dictate the resultant microstructures and properties of the processed materials, which then influence both part quality and performance. Open ended questions involving the optimization of process parameters for defect free and high performing parts remain at the heart of AM science. Further understanding of melt pool and solidification behavior is required to help answer these questions. In this presentation, 25 MHz ultrasonic shear waves are used to probe dynamic melt pool behavior in Al6061 by observing the scattering from the solid/liquid and liquid/gas boundaries. High-speed synchrotron x-ray imaging was employed simultaneously with the ultrasonic measurement for validation. Melt pool features observed in the x-ray images were then associated with features in the synced ultrasound measurement. Specifically, the scattered response tracked with the depth of the melt pool. Finite element models depicting wave-scattering were also used to predict wave scattering and help interpret the ultrasonic response. These preliminary results provide support for ultrasonic scattering as a promising method to evaluate melt pool behavior in metallic additive manufacturing processes.

Analysis of vapor plume and keyhole dynamics in laser welding stainless steel with beam oscillation

The influence of laser welding with beam oscillation on dynamic behavior of vapor plume, melt pool and keyhole was investigated. The stability of plume morphology is improved with a little fluctuation, and the height and area of plasma plume are decreased as the oscillation frequencies increased. The melt pool morphologies and flowing feature under beam oscillation conditions are also regulated due to the resultant energy deposition, which has an influence on weld solidification process. The re-melting effect of circular beam oscillation can eliminate pores formation and improve weld quality.

An Overview of Additive Manufacturing Technologies-A Review to Technical Synthesis in Numerical Study of Selective Laser Melting.

Additive Manufacturing (AM) processes enable their deployment in broad applications from aerospace to art, design, and architecture. Part quality and performance are the main concerns during AM processes execution that the achievement of adequate characteristics can be guaranteed, considering a wide range of influencing factors, such as process parameters, material, environment, measurement, and operators training. Investigating the effects of not only the influential AM processes variables but also their interactions and coupled impacts are essential to process optimization which requires huge efforts to be made. Therefore, numerical simulation can be an effective tool that facilities the evaluation of the AM processes principles. Selective Laser Melting (SLM) is a widespread Powder Bed Fusion (PBF) AM process that due to its superior advantages, such as capability to print complex and highly customized components, which leads to an increasing attention paid by industries and academia. Temperature distribution and melt pool dynamics have paramount importance to be well simulated and correlated by part quality in terms of surface finish, induced residual stress and microstructure evolution during SLM. Summarizing numerical simulations of SLM in this survey is pointed out as one important research perspective as well as exploring the contribution of adopted approaches and practices. This review survey has been organized to give an overview of AM processes such as extrusion, photopolymerization, material jetting, laminated object manufacturing, and powder bed fusion. And in particular is targeted to discuss the conducted numerical simulation of SLM to illustrate a uniform picture of existing nonproprietary approaches to predict the heat transfer, melt pool behavior, microstructure and residual stresses analysis.

Influence of gas atmosphere (Ar or He) on the laser powder bed fusion of a Ni-based alloy

The gaseous atmosphere plays a major role in the quality of the manufactured parts in Laser Powder Bed Fusion (L-PBF) by protecting the metal from high temperature oxidation. If argon and nitrogen are the most commonly used gases, helium has almost never been considered as a possible candidate as a chemically inert shielding gas. To provide a better understanding of the influence of the gas atmosphere on the process stability, a comparative study of L-PBF manufacturing under argon and helium atmospheres has been carried out, considering a nickel-based alloy Inconel® 625 and a single bead configuration. To this end, in-situ process measurements were carried out on a dedicated experimental setup. The melt pool behaviour, the expansion of the vapour plume and the amount of spatters were evaluated with high-speed imaging for the two gases considered, together with the final L-PBF bead dimensions. Results were also compared to single fusion beads carried out in an industrial L-PBF machine for a comparable range of volume energy densities. The influence of the shielding atmosphere on L-PBF single beads was as follows: (1) dimensions of beads were shown to be constant whatever the gas; (2) fewer and smaller spatters were produced under helium atmosphere, especially for high volume energy densities. Physical mechanisms were then discussed to understand those specific effects.

Multiphase mesoscopic simulation of multiple and functionally gradient materials laser powder bed fusion additive manufacturing processes

Multiple material laser powder bed fusion (MMLPBF) is a recently developed technology that broadens the application range of additive manufacturing. Experiencing dissimilar materials interactions, the physical phenomena in the MMLPBF process are more complex. In this study, a computation fluid dynamic model in the mesoscale for simulating single-track MMLPBF melt pool behavior is developed. The characteristics of melting IN718/Cu10Sn powder beds were investigated. It is shown that in melting a mixed IN718/Cu10Sn powder bed, the temperature of the melt pool decreases with increasing Cu10Sn fraction, owing to the difference in the thermal–physical properties of dissimilar metals, particularly the melting points, laser beam absorptivity and thermal conductivity. A braze zone is formed on the edge of a solidified track on the powder bed with mixed dissimilar powders having melted Cu10Sn and un-melted or partially melted In718, which results in fluctuations in the track dimensions. For unmixed dissimilar powder beds, an asymmetrical melt pool and solidified track are formed if the laser spot overlaps the interface of the dissimilar powders. The effect of hatch spacing in multiple track melting on a mixed dissimilar powder bed is also studied. It indicates that owing to the significant fluctuation in the track dimensions, the hatch spacing should be adjusted adaptively and should be less than the width of the melted zone to avoid insufficient intertrack fusion.

In-process measurement of melt pool cross-sectional geometry and grain orientation in a laser directed energy deposition additive manufacturing process

Understanding the behaviour of melt pool during laser directed energy deposition (L-DED) is essential for the prediction and control of process quality. Previous effort was focused on the observation of melt pool surface characteristics. In this paper, a coaxial imaging system was employed to determine the melt pool cross sectional geometry and to predict solidified grain orientation during a high deposition rate L-DED process. The image processing procedure, deposition track cross-sectional profile prediction and the relationship between melt pool shape and melt pool dynamics, and grain growth orientation were investigated. Results show that sharp melt pool edges can be obtained so that melt pool width can be predicted with an accuracy of more than 95%. The estimation method of melt pool length has an accuracy of 90%. With the experimental melt pool width and depth data, the cross-sectional profiles of deposited track are predicted at an accuracy of 92% and a good match with experimental data is obtained. The melt pool formation is found to be able to allow the prediction of crystal growth directions during solidification.

From Melt Pool to Microstructure to Mechanical Properties: A Simulative Approach to L-PBF Processed Material Behaviour

An important goal for the additive manufacturing (AM) community is to predict the material-dependent effective mechanical properties of a build-up specimen. In this work three coupled simulations to calculate the mechanical properties of laser-power bed fusion (L-PBF) processed IN718 are used. These values are compared to experimental data from literature. In the future, this approach is going be used for newly designed materials to estimate the expected mechanical properties and the resulting microstructure (As an extra step, the thermophysical properties of the new alloy have to be calculated in advance). The simulation chain to solve this challenge involves the simulation of the melt pool behaviour during the L‑PBF process to obtain the solidification conditions. These are required to then calculate the microstructural growth behaviour with the phase-field method. The resulting microstructure is homogenized to obtain the final effective mechanical properties of the L‑PBF processed material. The simulation results are compared with according results from experiments at each step to verify the approach and each simulation result.

Hierarchical spatial-temporal modeling and monitoring of melt pool evolution in laser-based additive manufacturing

Melt pool dynamics reflect the formulation of microstructural defects in parts during Laser-Based Additive Manufacturing (LBAM). The thermal images of the melt pool collected during the LBAM process provide unique opportunities for modeling and monitoring its evolution. The recognized anomalies are evidence of part defects that are to be eliminated for higher product quality. A unique concern in analyzing thermal images is spatial-temporal correlations – the heat transfer within the melt pool causes spatial correlations among pixels in an image, and the evolution of the melt pool causes temporal correlations across images. The objective of this study is to develop a LBAM modeling-monitoring framework that incorporates spatial-temporal effects in characterizing and monitoring melt pool behavior. Spatial-Temporal Conditional Autoregressive (STCAR) models are explored. STCAR-AR is identified as the best candidate among the numerous STCAR variants. A novel two-level control chart is constructed on top of the STCAR-AR model to monitor the melt pool dynamics. A hierarchical structure underlies the two-level control chart in the sense that global anomalies recognized in Level II can be traced in Level I for further inspection. A comparison with other recently developed in-situ monitoring approaches shows that the proposed framework achieves the best detection power and false positive rate.

Edge and corner effects in selective laser melting of IN 625 alloy

Experimental investigations on top surface of prismatic specimens, manufactured by Selective Laser Melting of IN 625 alloy, were carried out in order to assess the influence of laser power and scanning speed on edge and corner effects. Since the melt-pool behaviour is strongly influenced by the process parameters, all specimens were manufactured with no contour using the same layer thickness, hatch distance and scanning strategy at different levels of laser powers and scanning speeds. 3D laser surface scanning was performed in order to measure surface changes. The experimental results have revealed that melt-pool behaviour during solidification generates elevated ridges on both specimen sides and corners that are strongly influenced by the energy input. The edge ridges width increases with increasing the laser power and decrease with increasing the scanning speed, the rising of corners being much more pronounced. On the contrary, at constant laser power and variable scanning speeds the edge and corner ridges decrease.

An investigation upon overhang zones by using finite element modelling and in-situ monitoring systems

The characteristic of heat transfer in direct metal laser sintering is a key point in terms of overall material properties. Thus, this research focuses on the temperature distribution in overhang zones of Inconel 625 by employing finite element modelling and in-situ monitoring systems. The larger fluctuations in radiation signals are observed during solidification of melt pools. Furthermore, optical tomography confirms that overheating caused by lack of heat transfer between melt pool and powder occurs in overhang regions. This phenomenon is explained by finite element modelling based on thermal behavior of melt pool with powder, and surface characterization.

Heat transfer, fluid flow and electromagnetic model of droplets generation and melt pool behaviour for wire arc additive manufacturing

In this study, a numerical model of Wire Arc Additive Manufacturing has been developed to obtain the geometry of the part as well as its temperature field from the operating parameters. This predictive model takes into account electromagnetism, fluid flow and heat transfer in the arc and the melt pool. The Lorentz forces, shear stress, arc pressure, and Joule effect are calculated. This model is developed using the COMSOL Multiphysics® software. In order to simulate the addition of layer-by-layer material and the strong topological changes, the level set interface tracking method is used. This model aims to simulate the build-up of a 304 stainless steel rod starting from the operating parameters in the case of a pulsed currents. The detachment of droplets of the deposited metal and their fall along the vertical axis are modelled to predict the geometry and the thermal history of the workpiece. The "material supply / cooling" cycles between each layer are simulated. To validate this model, the geometry and the temperature field are analysed and compared to experimental data.

Experimental research on heat transfer behavior of large scale two-layer salt melt pool based on COPRA facility

During a core meltdown accident, the significant decay heat within the corium pool will continually impose thermal load on the vessel wall and challenge the integrity of reactor pressure vessel (RPV). External Reactor Vessel Cooling (ERVC) is considered to be an effective strategy to reduce the potential of vessel failure. After long-term cooling, the initially uniform melt pool can reach a final steady state with stable two-layer or three-layer configuration. Based on the COPRA facility, a series of tests were performed to study the natural convection heat transfer phenomenon of stratified corium pools. The test section was a full-scale 2D slice, simulating the hemispherical lower head of the HPR1000 reactor design. Direct electrical heating was applied in the lower part of vessel to simulate the homogenously heated ceramic pool, while no heat was generated in the upper part to model the metallic layer. A binary mixture of 20 mol% NaNO3–80 mol% KNO3 was selected as simulant material due to the similar phase-change characteristics with the prototypical melt. During the experiment, effects of heating power, top boundary condition, corium pool height, decay heat distribution and simulant material on corium pool heat transfer behavior were analyzed. The results implied that the top cooling and decay heat distribution slightly influenced the temperature and heat flux of ceramic pool, but strongly impacted the characteristics of metallic layer. The comparison between the dimensionless parameters of water tests and salt tests also demonstrated the key role of simulant material. Furthermore, the downward heat transfer Nudn and the upward heat transfer Nuup obtained from COPRA two-layer experiment were in accordance with the previous data or empirical correlations. Note that using the same simulant material for oxide layer and metal layer fails to repeat the boundary conditions in the reactor case, however, the current study pays more attention to the qualitative analysis of different influence on natural convection heat transfer behavior. More valuable information about the crust characteristics and stratification effect can be obtained by the following CFD simulation of these tests.

Comparative analysis of simulations of LIVE-L10 and -L11 experiments using different lower head modules of AC2

Abstract The stabilization of molten core material in the lower head in case of a severe accident by external cooling of the reactor vessel is regarded as an effective severe accident management measure. In the experiments LIVE-L10 and -L11 the late phase melt pool behaviour of the corium is investigated under different cooling conditions – the former under sub-cooled convection, the latter under nucleate boiling conditions. In this work the experiments are calculated with the severe accident analysis code AC2 – ATHLET-CD 3.1A. Objective of the simulations conducted is the analysis and assessment of the code's capability to simulate the most relevant phenomena that occur during the tests. The simulations are performed with two different lower head modules implemented in ATHLET-CD, AIDA (Analysis of the Interaction between Core Debris and the reactor pressure vessel during severe Accidents) and LHEAD (extended Lower Head module). The simulation results, analysed in comparison with the experimental results, show the capability of both modules to reproduce the respective experiments.

Influence of the Wire Feeding on the Wetting Process during Laser Brazing of Aluminum Alloys with Aluminum-Based Braze Material

The wetting behavior in laser brazing can be designated as inconstant, caused largely by external process discontinuities such as the wire feeding. To reveal periodic melt pool propagation effects that occur during laser brazing of aluminum and for a better understanding of those effects in laser brazing in general, this paper analyzes high-speed recordings of the brazing process with aluminum alloy. It is demonstrated that two main effects of periodic melt pool behavior in different frequency scales occur during the process, related directly to the wire feeding.

Melt pool geometry and morphology variability for the Inconel 718 alloy in a laser powder bed fusion additive manufacturing process

Expanding on prior process mapping work by the authors, multiple melt pool cross-sections are measured at multiple process parameter combinations for the Inconel 718 alloy in a Laser Powder Bed Fusion (L-PBF) process. Collection of such data enables the study of the variability of melt pool geometry (e.g. width, depth, and cross-sectional area) across process space. Furthermore, the statistical distribution of the measured melt pool geometries is compared to that of an equivalent normal distribution and intriguing outliers are observed. The cross-sectional morphology of the melt pools are associated with defects such as keyholing porosity and balling and the variability of the defects is quantified. The final product of this work is a robust description of L-PBF In718 melt pool behavior, based on ex-situ observations, which can be linked to in-situ observations of melt pool morphology in future work.

Assessment of PECM as an efficient numerical analysis tool for investigating convective heat transfer phenomena during PCM melting

• PECM as a numerical analysis tool, applied in FLUENT CFD as UDF. • The tool is used here to predict thermal hydraulic behavior of PCM melt pool. • The selected modelling approach is validated against SIGMA experiment. • The PECM simulation result is compared with FLUENT CFD as a benchmark test. • The tool is then applied for single layer PCM with different boundary conditions & heat source. • Finally, PECM is applied in two-layer PCM melt pool and compared to single layer. In the framework of this research work, the principle focus is to assess the applicability & reliability of the Phase change Effective Convectivity Model (PECM) as a numerical analysis tool to investigate natural convective heat transfer in single and two-fluid density PCM molten pools. The model is applied in ANSYS FLUENT as User Defined Function (UDF) to predict convective melt pool thermal hydraulics in a volumetrically heated PCM (Phase Change Material) melt pool. As a part of this work, PECM is tested first by a benchmark case against CFD to gain confidence in its applicability as an analysis tool. Two commercial PCMs: RT50 and C58, are introduced in a 3D semicircular vessel slice with their thermo-physical properties as input for modelling. The sidewalls made of quartz glass are used for direct visualization of convective heat transfer phenomena. It is ensured that the conditions of nearly constant density of power deposition over the entire volume of the PCM melt pool throughout the series of simulation cases. The values of characteristic numbers ranged within the following limits with different pool height corresponding modified Rayleigh number Ra=1012-1013 and for Prandtl number Pr=5-7. The selected modelling approach is validated against SIGMA experiment with respect to the angular distribution of heat flux that qualify our model to run in the proceeding calculation using PECM. Following benchmark test results of PECM compared with that of conventional enthalpy porosity method embedded in ANSYS FLUENT, PECM is applied in 1-layer and 2-layer PCM configuration to study in details of the influence of different boundary conditions, internal heat sources (QV) and heat transfer fluid (HTF) cooling condition to quantify the thermal loads. Finally, the comparison is made between two PCM configurations in terms of the quantification of the thermal load to justify PECM as an efficient numerical analysis tool for investigating convective heat transfer phenomena during PCM melting.

In situ X-ray imaging of melt pool dynamics in underwater arc welding

The dynamic behavior of the underwater melt pool was firstly in-situ observed with the X-ray transmission method. A method for quantitatively characterizing the melt pool stability was pioneered. Comparing with conventional melt pool composing metal liquid, the pool was filled with bubbles in underwater environment. By observing the X-ray images and charactering the weld pool waveform, it is confirmed that the underwater melt pool was more volatile than onshore melt pool, which was mainly caused by the evolution of large bubble in the pool. Violent fluctuation of melt pool was the basic reason gaining poor weld and multiple defects in the underwater wet welding. To mitigate pool fluctuations by reducing the size of bubble, the ultrasonic vibration was applied directly on the workpiece, which improved the melt pool stability, uniformized the weld formation, and decreased the weld porosity.

Influence of the vapour channel on processing in laser powder bed fusion

Additive Manufacturing provides many opportunities to design and manufacture parts that are difficult or not possible to produce with conventional methods. In Selective Laser Melting (SLM) in powder bed fusion (PBF), melt pool dynamics and stability is dependent on a large number of factors, e.g. laser power output, power density, travel speed, reflectivity of powder bed, rapid heating and vaporization. Since travel speeds are often very fast and the laser interaction zone is small, the physical events become difficult to predict but also to observe. This work aims to describe the formation and geometrical characteristics of the vaporization zone during processing. Using a combination of theoretical descriptions, resulting material structures and a comprehensive analysis of high-speed images of the processing zone for different heat inputs and travel speeds, explanations for the dynamic melt pool behaviour are derived. The melting and pressures from processing involved moves powder particles next to it, changing the conditions for neighbouring tracks due to lack of material. These findings can provide a basis for creating more efficient and stable SLM processing, with fewer imperfections.

Experimental and numerical investigation of molten corium behavior in lower head under external subcooling and boiling conditions

The in-vessel melt retention by flooding the reactor vessel externally is regarded as an effective severe accident management (SAM) strategy. According to this strategy, the corium will be stabilized within the lower head, by transferring the decay heat through the wall into the containment via external cooling. One key question of this strategy is how the melt pool heat transfer reacts to different external cooling conditions. In this paper, the melt’s thermal–hydraulic behavior under different external cooling conditions is studied experimentally in two LIVE tests performed in the frame of the LIVE program investigating late in-vessel melt pool behavior and calculated with the lower head module AIDA of ATHLET-CD. One LIVE test was performed under nucleate boiling condition, the other under sub-cooling condition. Melt temperature, heat flux along the curved vessel wall and the crust behavior are described in transient and steady states. The simulation results have been compared with the experimental results. The results have been demonstrated the applicability of ATHLET-CD to investigate the SAM strategy in-vessel melt retention by external cooling. Furthermore, on the basis of the experimental results the modelling of heat transfer between corium and coolant has been improved.

COPRA experiment and numerical research on the behavior of internally-heated melt pool with eutectic salt

In-vessel retention (IVR) has been proposed as an effective mitigation strategy during severe accidents in pressurized water reactors (PWRs). To help understand better the application of IVR, the large-scale COPRA facility was used for experimental research and the eutectic salt, i.e. the binary mixture of 50 mol% NaNO3 – 50 mol% KNO3 was selected as the simulant material for corium. Besides, numerical studies were done to capture the detailed flow field information. The experiment studied the effects of a sudden change in boundary condition and the influences of power changes on the transient behaviors and the steady-state characteristics of melt pool. Experimental results show that a sudden introduction of sidewall cooling and the melt pouring are the most threatening factors to the reactor vessel safety, but which can be guaranteed as long as the vessel is kept undamaged during melt pouring phases. Also, power changing won’t lead to rapid variation of sideward heat flux, so the correlations for steady state heat transfer calculation are basically qualified to evaluate the reactor safety in analysis of long time cooling. Besides, the power changing only exerts obvious impacts on the crust growth on the curved sidewall inner surface with polar angle below 30°, contrary to which the sideward heat flux is clearly influenced by different power densities only above the angle of 30°. Finally, the numerical simulation results show that vortexes could form near the region where the crust formed, and lead to a more homogeneous temperature distribution; generally, the convection happens between the lower part of melt pool and cooling boundary, with flow velocity at a magnitude of 0.01 m/s.