Melt Pool Characteristics Research Articles

Study and modeling of melt pool evolution in selective laser melting process of SS316L

Abstract

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.

Porosity prediction: Supervised-learning of thermal history for direct laser deposition

The objective of this study is to investigate the relationship between the melt pool characteristics and the defect occurrence in an as-built additive manufacturing part. One of the major detrimental microstructure properties associated with additive manufacturing (AM) is porosity within final parts. State-of-the-art porosity detection methods focus primarily on post-manufacturing approaches that are susceptible to high cost of process, longer process time, and are incapable of characterizing pores during fabrication. A real-time porosity prediction method is developed using morphological characteristics of the melt pool boundary (i.e., features obtained via functional principal component analysis (FPCA)). A thermal monitoring system is used to capture the time-varying melt pool signal, which are labeled as either pores or normal melt pools by X-ray tomography. Supervised learning methods are utilized to identify the patterns of melt pool images and build a black-box model for the probability distribution of class labels (namely, porosity) based on data characteristics of predictors (e.g., melt pool characteristics). The resultant model does not depend on specific design of specimens with varying material properties; and can be effectively developed as long as thermal-porosity data can be obtained. In the current study, multiple supervised machine learning approaches are used to classify melt pools to predict porosity in a part. Two different accuracy measures are used and numerical experiments show that among the classification approaches used (i.e., Decision Tree (DT), K-Nearest Neighbor (KNN), Support Vector Machine (SVM), Linear Discriminant Analysis (LDA), and Quadratic Discriminant Analysis (QDA)), KNN results in the highest rate of accurately classifying melt pools (98.44%). However, DT results in the lowest rate for incorrectly identifying normal melt pools as pores (0.03%). A comparative study is conducted that compares the performance of supervised learning methods leveraging the proposed morphological model and simple metrics of the melt pool. Numerical experiments show that the morphological model combined with supervised learning techniques vastly outperform the simple melt pool metrics combined with supervised learning techniques (approximately 250% better performance for correctly predicting abnormal melt pools). Our approach may potentially be applied to other AM processes that share similar energy-material interactions (e.g., powder bed fusion, electron beam melting).

Analysis of transient thermo-fluidic behavior of melt pool during spot laser welding of 304 stainless-steel

In this study, the transient evolution of temperature and velocity field, melt pool geometry (width, depth of penetration and melt pool volume), melting and solidification behavior during spot laser welding of 304 stainless-steel is presented. The transport phenomena associated with the melting and the solidification stage are appropriately accounted in the computational model. The predicted melt pool geometry shows a good agreement with the published and in-house experimental data for spot laser welding. Analysis of important dimensionless numbers revealed the significant role of melt pool convection on the heat transfer and the melt pool geometry. Thermal cycles at several strategic locations were also investigated during the melting and the solidification stage. To analyze the solidification behavior of the melt pool, the relevant solidification parameters such as temperature gradients, solidification rates, and cooling rates were described. With progress in solidification of the melt pool at the melt pool interface, the solidification rate increases while the temperature gradient decreases. Also, an attempt has been made to account the free surface deformation in the melt pool by developing a separate numerical model considering the free/deformable top surface. A qualitative and quantitative comparison of the melt pool characteristics obtained from the free/deformable and the flat top surface model has been carried out. The predicted depth of penetration of the melt pool with the free/deformable top surface is found to be slightly larger than that with the flat top surface.

Effect of Laser Power and Scan Speed on Melt Pool Characteristics of Commercially Pure Titanium (CP-Ti)

Selective laser melting (SLM) is an additive manufacturing technique that creates complex parts by selectively melting metal powder layer-by-layer using a laser. In SLM, the process parameters decide the quality of the fabricated component. In this study, single beads of commercially pure titanium (CP-Ti) were melted on a substrate of the same material using an in-house built SLM machine. Multiple combinations of laser power and scan speed were used for single bead fabrication, while the laser beam diameter and powder layer thickness were kept constant. This experimental study investigated the influence of laser power, scan speed, and laser energy density on the melt pool formation, surface morphology, geometry (width and height), and hardness of solidified beads. In addition, the observed unfavorable effect such as inconsistency in melt pool width formation is discussed. The results show that the quality, geometry, and hardness of solidified melt pool are significantly affected by laser power, scanning speed, and laser energy density.

Influence of Powder Bed Characteristics on Material Quality in Additive Manufacturing

In powder bed based Additive Manufacturing (AM) processes like Selective Laser Melting (SLM) or Electron Beam Melting (EBM), the spatial distribution of the individual powder particles is typically unknown. Nevertheless, the distribution of particles in the heat affected zone defines the thermophysical properties of the region being processed by the heat source and therefore plays a crucial role in heat transfer processes. In this work, the spatial distribution of individual particles and their influence on the AM process is numerically investigated. Two powder bed configurations are compared: One powder bed is generated using the discrete element method (DEM) to model the coating process; the second powder bed is arranged in the BCC structure. The melting and solidification of both configurations are modelled. The predicted melt pool dimensions are compared with experimentally determined values. The results indicate that modelling the coating process is necessary to ensure accurate modelling of the heat source powder bed interaction as well as an accurate prediction of the melt pool characteristics.

Prediction of microstructure in laser powder bed fusion process

Additive manufacturing (AM) processes are receiving widespread attention due to the ability to create or repair precision engineering components without use of any die or mold. Currently, the approach to obtain a specific user defined/as-desired or conformal/epitaxial microstructure is a challenging and expensive iterative process. Modeling and validation of solidification microstructure can be leveraged to reduce iteration cost in obtaining a desired microstructure. Numerical Volume-of-fluid based method incorporating Marangoni convection can accurately predict the resultant melt pool geometry and temperature distribution which can serve as an input in prediction of microstructure evolution in solidifying mushy region. Hence, in the present study, computational fluid dynamics (CFD) analysis is used to predict melt pool characteristics and phase field modeling is employed to simulate microstructure evolution in the as-deposited state for laser powder bed fusion (LPBF) process. Different features of LPBF microstructure such as segregation of secondary elements, dendrite sizes, dendritic orientation, dendritic morphology, and surface roughness are investigated and validated through comparison with experimental results. Phase-field model suggests strong dependency of dendrite orientation on surface roughness and scan speed and suggests potential of columnar flip or oriented-to-misoriented transition at higher scan speed. Segregation of the secondary elements is found to be the dominant factor in resultant dendrite width in the range of 1–3 μm. Furthermore, the developed method can easily be extended to predict the change in orientation of dendrites as new layers are built atop previous layers.

A numerical investigation of thermal property effects on melt pool characteristics in powder-bed electron beam additive manufacturing

Powder-bed electron beam additive manufacturing has the potential to be a cost-effective alternative in producing complex-shaped, custom-designed metal parts using various alloys. Material thermal properties have a rather sophisticated effect on the thermal characteristics such as the melt pool geometry in fabrications, impacting the build part quality. The objective of this study is to achieve a quantitative relationship that can correlate the material thermal properties and the melt pool geometric characteristics in the electron beam additive manufacturing process. The motivation is to understand the interactions of material property effect since testing individual properties is insufficient because of the change of almost all thermal properties when switching from one to the other material. In this research, a full-factorial simulation experiment was conducted to include a wide range of the thermal properties and their combinations. A developed finite element thermal model was applied to perform electron beam additive manufacturing process thermal simulations incorporating tested thermal properties. The analysis of variance method was utilized to evaluate different thermal property effects on the simulated melt pool geometry. The major results are summarized as follows. (1) The material melting point is the most dominant factor to the melt pool size. (2) The role of the material thermal conductivity may outweigh the melting point and strongly affects the melt pool size, if the thermal conductivity is very high. (3) Regression equations to correlate the material properties and the melt pool dimension and shape have been established, and the regression-predicted results show a reasonable agreement with the simulation results for tested real-world materials. However, errors still exist for materials with a small melt pool such as copper.

Simulation of metallic powder bed additive manufacturing processes with the finite element method: A critical review

This article provides a literature review of finite element simulation studies for metallic powder bed additive manufacturing processes. The various approaches in the numerical modeling of the processes and the selection of materials properties are presented in detail. Simulation results are categorized according to three major findings’ groups (i.e. temperature field, residual stresses and melt pool characteristics). Moreover, the means used for the experimental validation of the simulation findings are described. Looking deeper into the studies reviewed, a number of future directions are identified in the context of transforming simulation into a powerful tool for the industrial application of additive manufacturing. Smart modeling approaches should be developed, materials and their properties should be further characterized and standardized, commercial packages specialized in additive manufacturing simulation have to be developed and simulation needs to become part of the modern digital production chains. Finally, the reviewed studies are organized in a table and characterized according to the process and material studied, the modeling methodology and the experimental validation method used in each of them. The key findings of the reviewed studies are also summarized.

Macroscopic simulation and experimental measurement of melt pool characteristics in selective electron beam melting of Ti-6Al-4V

Selective electron beam melting of Ti-6Al-4V is a promising additive manufacturing process to produce complex parts layer-by-layer additively. The quality and dimensional accuracy of the produced parts depend on various process parameters and their interactions. In the present contribution, the lifetime, width and depth of the pools of molten powder material are analyzed for different beam powers, scan speeds and line energies in experiments and simulations. In the experiments, thin-walled structures are built with an ARCAM AB A2 selective electron beam melting machine and for the simulations a thermal finite element simulation tool is used, which is developed by the authors to simulate the temperature distribution in the selective electron beam melting process. The experimental and numerical results are compared and a good agreement is observed. The lifetime of the melt pool increases linearly with the line energy, whereby the melt pool dimensions show a nonlinear relation with the line energy.

Single scan vector prediction in selective laser melting

In selective laser melting (SLM) products are built by melting layers of metal powder successively. Optimal process parameters are usually obtained by scanning single vectors and subsequently determining which settings lead to a good compromise between product density and build speed. This paper proposes a model that describes the effects occurring when scanning single vectors. Energy absorption and heat conduction are modeled to determine the temperature distribution and melt pool characteristics for different laser powers, scan speeds and layer thicknesses. The model shows good agreement with experimentally obtained scan vectors and can therefore be used to predict SLM process parameters.

Numerical study on the heat transfer characteristics of LIVE-L4 melt pool with a partial solidification process

Using the source-based SIMPLE algorithm based on a fixed grid method, a two-dimensional numerical model for a convection-diffusion controlled mushy region phase-change problem was developed to investigate the heat transfer characteristics of LIVE L4 melt pool subjected to a partial solidification process in a Pressurized Water Reactor (PWR) lower plenum during a hypothetical severe accident. For the binary non-eutectic mixtures of L4 melt, a linear liquid fraction temperature relationship was implemented on the calculations of the velocity and enthalpy in the mushy zone. The effect of fluid flow in the melt pool was analyzed, and numerical results for the cases with and without phase change model were calculated to investigate the effects of solidification on the heat transfer characteristics of L4 melt pool. Numerical results indicated that the phase-change model could well predict the main parameters of melt pool, e.g. the melt pool temperatures, heat flux through the melt pool, and the crust thickness. Results also indicated that the predicted Nu number without solidification was overestimated by about 12%, compared with that with solidification.

Laser Brazing with Beam Scanning: Experimental and Simulative Analysis

Laser beam brazing with copper based filler wire is a widely established technology for joining zinc-coated steel plates in the body-shop. Successful applications are the divided tailgate or the zero-gap joint, which represents the joint between the side panel and the roof-top of the body-in-white. These joints are in direct view to the customer, and therefore have to fulfil highest optical quality requirements. For this reason a stable and efficient laser brazing process is essential. In this paper the current results on quality improvement due to one dimensional laser beam deflections in feed direction are presented. Additionally to the experimental results a transient three-dimensional simulation model for the laser beam brazing process is taken into account. With this model the influence of scanning parameters on filler wire temperature and melt pool characteristics is analyzed. The theoretical predictions are in good accordance with the experimental results. They show that the beam scanning approach is a very promising method to increase process stability and seam quality.

Effects of Melt Pool Variables and Process Parameters in Laser Direct Metal Deposition of Aerospace Alloys

Despite considerable advances in of laser direct metal deposition (LDMD) process optimization, there is rather limited work reported on the effects of melt pool variables on the final deposit characteristics. This article considers the effects of process parameters and melt pool characteristics on the deposition of Inconel 718 powder on a Ti-6Al-4 V thin wall. A 1.5 kW diode laser and LDMD system is used to produce a series of deposits. Images of the process are captured using Cu-vapor laser illumination and a high speed camera with long range microscopy optics, and quantitative results are extracted via image analysis. Process parameters such as carrier gas flow rate, powder mass flow rate and laser operating mode (CW and pulsed) and in process variables such as quantified melt pool disturbance, and final part characteristics are correlated and discussed. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and energy dispersive X-ray spectroscopy (EDS) are used to analyze deposited clads in terms of elemental composition and flow characteristics in the deposition melt pool. Melt pool disturbance is found to be a vital parameter in determining the surface roughness of the final part. An inverse relation between the mean surface disturbance of the melt pool and the surface roughness of the part is observed, and carrier gas flow rate and powder mass flow rate both affect the overall melt pool size. The work has implications for the selection of process parameters for commercial laser deposition processes–the speed with which powder is delivered to the melt pool as well as the mass flow rate may need to be taken into account when calculating build rate and for a good surface finish requiring minimum post process finishing a stable melt pool may actually be the worst situation.

FEM modeling and experimental verification for dilution control in laser cladding

A model for the determination of the clad geometry and dilution in laser cladding is presented. The model uses a novel approach to determine the clad geometry. This involves a mathematical transformation of the balance equations describing the physical effects. The correlation between observable melt pool characteristics and dilution is investigated using this model. Simulations were performed for different combinations of cladding speed, laser power (distribution) and substrate temperature. Simulation results are compared to experimental results, showing a high degree of agreement. A high correlation between melt pool width and dilution was found, which was almost independent of the process settings and substrate temperature. This makes the width an excellent sensor for online dilution control.

Investigation into the effect of beam shape on melt pool characteristics using analytical modelling

An established analytical model is used to simulate an extended laser beam. Multiple Gaussian sources are superimposed to form a rectangular beam and results are compared with a single circular Gaussian source model as well as experimental results from a high power diode laser with a rectangular beam. Melt depth and melt pool profile and progression have been predicted by modeling, which are compared with experimental results from melting of Inconel 625. The model produced is shown to give a reasonable prediction of melt pool shape and can be usefully employed to help optimise overlap required for laser surface processing applications. The value of absorptivity used in the model can be used as a fitting parameter to optimise the match between experimental and predicted results.

Rapid solidification and metallic glass formation – Experimental and theoretical limits

Most of the metallic glasses which have been identified to date require rapid solidification processing (RSP) in order to obtain the high cooling rates required to avoid the thermally activated decomposition reactions that result in the formation of crystalline phases. While the free-jet melt spinning technique is commonly used for the investigation of amorphous alloy formation, many of the key process parameters remain poorly understood. In the work presented here, we utilize high speed digital imaging to investigate various melt-pool characteristics and their influence on the melt-spun ribbon. With detailed measurements of melt-pool geometry and surface temperature, we show that the length of this molten zone establishes a critical length scale which characterizes the ribbon thickness and cooling rate. Within certain limits, the ribbon thickness is shown to correlate with a simple 2D laminar flow boundary layer formed at the base of the melt-pool. Estimates of initial quench rate, based on surface temperature measurements, clearly reveal the importance of two distinct cooling periods. The first is the wheel-contact period, where the ribbon remains in intimate contact with the quench wheel and cooling is primarily governed by the heat conduction across the ribbon-wheel boundary. The second is the free-flight period, where the ribbon separates from the wheel, and cooling is governed by radiation and convection into the chamber.

Numerical analysis of the effects of non-conventional laser beam geometries during laser melting of metallic materials

Laser melting is an important industrial activity encountered in a variety of laser manufacturing processes, e.g. selective laser melting, welding, brazing, soldering, glazing, surface alloying, cladding etc. The majority of these processes are carried out by using either circular or rectangular beams. At present, the melt pool characteristics such as melt pool geometry, thermal gradients and cooling rate are controlled by the variation of laser power, spot size or scanning speed. However, the variations in these parameters are often limited by other processing conditions. Although different laser beam modes and intensity distributions have been studied to improve the process, no other laser beam geometries have been investigated. The effect of laser beam geometry on the laser melting process has received very little attention. This paper presents an investigation of the effects of different beam geometries including circular, rectangular and diamond shapes on laser melting of metallic materials. The finite volume method has been used to simulate the transient effects of a moving beam for laser melting of mild steel (EN-43A) taking into account Marangoni and buoyancy convection. The temperature distribution, melt pool geometry, fluid flow velocities and heating/cooling rates have been calculated. Some of the results have been compared with the experimental data.

Metallographic Investigations of Solidification Microstructures and Melt-Pool Characteristics in Oriented Single Crystals of Stainless Steel

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005