Melting Of Bed Research Articles

A greedy algorithm for optimal heating in powder-bed-based additive manufacturing

Powder-bed-based additive manufacturing involves melting of a powder bed using a moving laser or electron beam as a heat source. In this paper, we formulate an optimization scheme that aims to control this type of melting. The goal consists of tracking maximum temperatures on lines that run along the beam path. Time-dependent beam parameters (more specifically, beam power, spot size, and speed) act as control functions. The scheme is greedy in the sense that it exploits local properties of the melt pool in order to divide a large optimization problem into several small ones. As illustrated by numerical examples, the scheme can resolve heat conduction issues such as concentrated heat accumulation at turning points and non-uniform melt depths.

A multiphysics simulation approach to selective laser melting modelling based on cellular automata and lattice Boltzmann methods

This paper presents a novel approach to numerical modelling of selective laser melting (SLM) processes characterized by melting and solidification of the deposited particulate material. The approach is based entirely on two homogeneous methods, such as cellular automata and Lattice Boltzmann. The model components operate in the common domain allowing for linking them into a more complex holistic numerical model with the possibility to complete full-scale calculations eliminating complicated interfaces. Several physical events, occurring in sequence or simultaneously, are currently considered including powder bed deposition, laser energy absorption and heating of the powder bed by the moving laser beam leading to powder melting, fluid flow in the melted pool, flow through partly or not melted materials and solidification. The possibilities and benefits of the proposed solution are demonstrated through a series of benchmark cases, as well as model verifications. The presented case studies deal mainly with melting and solidification of the powder bed including the free surface flow, wettability, and surface tension. An example of process simulation shows that the approach is generic and can be applied to different multi-material SLM processes, where energy transfer including solid–liquid phase transformation is essential, by integrating the developed models within the proposed framework.

Additive Manufacturing of Multi Layered Bioactive Materials with Improved Mechanical Properties: Modelling Aspects

Multilayered laminate structures obtained by coating of ultrafine-grained metallic materials with bioactive and multifunctional composite coatings are considered for biomedical applications. Laser-assisted densification of multiple materials using laser cladding and selective laser melting is an alternative route to reduce the risk of early implant failure allowing for faster and cheaper fabrication. To understand the cooperative relationships between different factors that cam influence the manufacture of such bioactive laminates reflecting in their bioactivity and mechanical properties, the multi scale numerical modelling is applied. This work presents resent advances on development of integrated numerical models including generation, melting and solidification of the powder bed, considering surface flow, wettability, surface tension and other physical phenomena, specific mechanical and thermo-mechanical aspects and microstructure evolution.

Grain growth prediction in selective electron beam melting of Ti-6Al-4V with a cellular automaton method

An integrated modeling framework coupling the discrete element method for powder spreading, finite volume method for powder bed melting, and an extended cellular automaton method for grain structure evolution during solidification is proposed. In this framework, the initial grain structure of both the substrate and metal powders can be taken into account and used to capture epitaxial and competitive grain growth. The framework is used to provide an in-depth understanding of microstructure development in Ti-6Al-4V during the selective electron beam melting process. The complex process of grain growth during deposition of multiple tracks and multiple layers is modeled through an analysis restarting scheme. The epitaxial growth of grains from pre-existing grains, in particular the grains of partially melted powders, is reproduced. The mechanism of microstructure development within the overlap region of consecutive tracks and layers for various scan strategies is revealed. The simulation results are in qualitative agreement with experimental observation in the literature. The proposed modeling framework is a powerful tool to guide optimal process parameters that lead to designed, site-specific microstructure control and therefore to tailored mechanical properties of parts fabricated by the powder bed fusion additive manufacturing process.

Melting Behavior of Heterogeneous Polymer Bulk Solids Related to Flood Fed Single Screw Extruders.

Melting models for flood fed single screw extruders, like the Tadmor model, describe the melting of pure thermoplastic polymers. However, the melting behavior of heterogenous polymer systems is of great interest for recycling issues, for example. In this work, the melting of polymer mixtures and that of pure bulk polymers by the drag induced melt removal principle is examined both theoretically and experimentally. The applied model experiments represent the melting of the solid bed at the barrel in single screw extruders. As polymer pellet mixtures, polypropylene-homopolymer mixed with polypropylene-block-copolymer, high density polyethylene, polyamide 6, and polymethylmethacrylate were studied using different mixing ratios. The melting rate and the shear stress in the melt film were evaluated dependent on the mixing ratio. The results show that when processing unfavorable material combinations, both shear stress and melting rate can be far below that of pure materials, which was also confirmed by screw extrusion and screw pull-out experiments. Furthermore, approaches predicting the achievable melting rate and the achievable shear stress of polymer mixtures based on the corresponding values of the pure materials are presented.

A preliminary study on concrete ablation during ex-vessel cooling in pre-flooded Westinghouse 3-loop NPP reactor cavity

We develop an analytical model about concrete ablation during ex-vessel cooling in a pre-flooded cavity. The model considers the melting of dry debris bed, the formation of crust, the growth of crust and the thermal ablation of concrete. This model can obtain concrete ablation depth during ex-vessel cooling. It is evaluated the adequacy of ex-vessel cooling in the pre-flooded reactor cavity of Westinghouse 3-loop nuclear power plants. If the debris bed is formed only in the zone directly below the reactor vessel narrowly and thickly, the characteristic of the debris bed should be like POMECO BED-5 and the pressure should be 2 bar or above to keep the integrity of the containment. However, if the debris bed is evenly distributed on the reactor cavity, it is assessed the pressure should be equal to or greater than 1 bar, 2 bar and 3 bar for POMECO BED-5, STYX BED and COOLOCED-8 BED, respectively.

Effect of shielding conditions on bead profile and melting behaviour in laser powder bed fusion additive manufacturing

A series of experiments were performed using a 500 W continuous wave fibre laser on a single powder bed layer using different processing variables. The aim was to investigate the effect of different shielding conditions on melting behaviour and bead profile in laser powder bed fusion (PBF). Through high-speed imaging, it was found that under an argon atmosphere a strong plasma plume is generated from the meltpool. Laser beam-plasma plume interactions caused strong instabilities during melting, including laser wandering, track instability and continuous fluctuations between melting regimes (conduction and keyhole). Hence, it was not possible to control the profile of the melted tracks under this condition. By using a helium atmosphere, a smaller plasma was obtained, reducing the disruptions caused by laser-plasma interactions. This led to a stable melting regime that allowed control of the melt bead profile. This condition was used to study the effect of laser-material fundamental interaction parameters on the bead geometry in powder bed melting. It was found that during melting of single tracks, the dominant regime of melting is conduction for the range of parameters tested. Penetration and melt width were found to increase with increasing energy density. For longer interaction times, melt widths were found to be up to ten times the size of the beam diameter used. Fluid flow modelling showed that this is due to strong melt flow as consequence of surface tension gradients generated by very high temperature gradients.

Selective laser melting of aluminum nano-powder particles, a molecular dynamics study

A quasi-2D model of Micro-selective laser melting (μ-SLM) process using molecular dynamics is developed to investigate the localized melting and solidification of a randomly-distributed Aluminum nano-powder bed. One of the biggest challenges in modeling the μ-SLM process is the computational treatment of the formation and growth of crystal nuclei in the meltpool. The present work overcomes this challenge using molecular dynamics simulation because of its capability to explicitly model the nucleation and growth of grains inside the meltpool. The localized heating and rapid solidification of meltpool is simulated by the direct control of the temperature in the meltpool both spatially and temporally. The rapid solidification in the meltpool reveals the cooling rate dependent homogeneous nucleation of equiaxed grains at the center of the meltpool. Additionally, the epitaxial grain growth from the adjacent laser tracks, previous layers, and partially melted nano-powders into the solidifying meltpool is observed along the highest heat flow directions. The growth of the long columnar grains into the top layer is inhibited if the penetration depth during the remelting of a previous layer is less than the depth of the equiaxed grains. Long columnar grains that spread across three layers, equiaxed grains, nano-pores, twin boundaries, and stacking faults are observed in the final solidified nanostructure obtained after ten passes of the laser beam on three layers of Aluminum nano-powder particles. Hot isostatic pressing (HIP) of the final solidified nanostructure is employed to eliminate the nano-pores, which act as sources of crack initiation during tensile loading. Higher applied stress is required for the crack initiation and propagation in the hot isostatically pressed nanostructure compared to the as-built Aluminum nanostructure owing to the absence of nano-pores.

Experimental Study of Thermomechanical Processes: Laser Welding and Melting of a Powder Bed

In this study, an experimental approach was developed to analyze and better understand the laser welding and melting of a powder bed process. Different optical diagnostics tools (high-speed camera, infrared camera, pyrometer, etc.) were applied to measure different physical quantities (molten pool morphology, temperature field, residual stresses, and distortions). As a result, measurements during the laser welding process facilitated the building of a database of experimental results (experimental benchmarks). The study of the melting of a powder bed enabled a better understanding of the physics related to the formation and behavior of the molten pool. These results can be used by researchers to improve and validate numerical simulations of these processes.

Threshold conditions for thermocapillary transition to deep penetration mode in selective laser melting of metal powder bed

The keyhole mode of selective laser melting (SLM) of metal powder bed in additive technology is characterized by an intensive hydrodynamic process in a thin molten layer. Such a mode is widely used also in laser and electron beam welding indicating the similarity of hydrodynamic processes in these technologies despite a significant difference of operating parameters. The threshold conditions of thermocapillary keyhole mode transition for various metals (Cu, Fe, Ti) in a wide range of beam parameters used for selective laser melting of metal powder layer and laser welding are investigated. The condition of threshold for thermocapillary transition into keyhole mode by sticking of viscous layer to the solid boundary is formulated. The fulfillment of this condition is confirmed by the convergence of estimated values of the viscous layer thickness and the molten layer depth during the transition to a keyhole mode. Analytical estimates of keyhole threshold and comparisons with experimental values of beam power and spot size corresponding to the transition in keyhole mode for SLM processes and laser welding are presented. The correlation of these values confirms the thermocapillary mechanism of cavity formation and the similarity of hydrodynamic processes in laser welding and SLM processing in keyhole mode at wide range of operating parameters.

On the phase field model for microstructure evolution during selective laser melting of powder bed including diffusion-induced stresses

In this paper, we develop a phase field model for microstructure evolution in laser powder-bed additive manufacturing processes, which allows for the effect of elastic stresses due to diffusion of atomic defects (vacancy and interstitial atom) generated at the moving liquid-solid interface. The governing equations of the model including a time-dependent Ginsburg-Landau equation for the phase field with additional stress term, a diffusion-drift Cahn–Hilliard like equation describing atomic defect dynamics, an energy balance equation for the temperature change, and finally the mechanical equilibrium equation for the elastic displacement fields are derived within a thermodynamic framework based on entropy generation guaranteeing thermodynamic consistency. The most simple 1D and isothermal equilibrium situation is considered to study phenomenon of diffusion-flexural instability with the atomic defect clustering and periodic distributions of defects. The effect of this instability on the exfoliation of deposited thin layers on a substrate is also discussed.

Production of Syngas Suitable to be Used in Fermentation to Obtain Biochemical Added-Value Compounds

Lignin-rich residues are a side stream in advanced ethanol biorefineries and they are usually used for heat and power production. However, an alternative is the lignin valorisation by thermochemical and biochemical processes that might be economically more suitable. In this work lignin-rich residues were firstly thermochemical converted to syngas in a fluidised bed gasifier, using steam and oxygen as gasifying agents. The obtained results so far showed that the rise of steam/lignin-rich residues ratio favoured the release of H2 and CO2 and led to a decrease in CO and hydrocarbons content, as steam reforming reactions were favoured. On the other hand, the rise of the equivalent ratio clearly favoured partial oxidation reactions and thus the release of CO and CO2, while H2 and hydrocarbons contents were reduced, due to their partial combustion. The increase of temperature favoured the formation of H2, at the expenses of hydrocarbons and tar release, as both decreased. However, it is fundamental to control gasification temperature to prevent ash melting and consequent bed agglomeration, due to the high ash content in lignin-rich residues, generally with high content of silica and alkali metals. The results led to the selection of gasification conditions (800 °C, equivalent ratio of about 0.13 and steam/lignin ratio of around 0.4) that headed to the following gas ratio CO/CO2/H2=1:0.7:0.7, which was tested for further bacterial fermentation. Fermentation tests with Butyribacterium methylotrophicum showed that the above syngas composition strongly influenced the production levels of acetic and butyric acids. Marked differences in the CO, H2 and CO2 consumption rates were also observed and were correlated with cell growth and metabolites production.

Melting Behaviour of Iron Ore Pellet Bed under Nut Coke Mixed Charge Conditions

The melting and dripping behaviour of an iron ore pellet bed mixed with nut coke are investigated through a series of quenching, melting and dripping experiments. In the melting bed of iron ore pellets, nut coke acts as a frame to maintain the passage for the gas flow. The iron carburisation level of the pellet shell is found to control the melting temperature of the pellet bed. Simultaneous and layer-wise melting is observed for the pellet bed with and without mixed nut coke, respectively. In the case of pellet bed mixed with nut coke, the liquid dripping starts at a lower temperature (1500°C) compared to the case when nut coke is absent (1518°C). Subsequently, a steady rate of liquid dripping is observed for the pellet bed mixed with nut coke. However, in the case of the pellet bed without nut coke, most of the liquid drips (~50 wt%) at high temperature (1550°C). The difference in carbon content of the quenched pellets and the dripped metal reveals that a substantial iron carburisation occurs when liquid iron flows over the regular coke particles. The nut coke is noticed to consumed preferentially in place of the regular coke. Additionally, the total coke consumption decreases with an increase in nut coke addition in the pellet bed. These results give support for more extensive use of nut coke as a replacement of the regular coke in the ironmaking blast furnace.

Effects of radiative local heating on metal solidification during selective laser melting for additive manufacturing

Selective laser melting (SLM) is a promising additive manufacturing technique arising from glassy metal characteristics of treated medium. When creating novel compositions of materials and intricate workpieces, reliable thermal designs should be implemented based on well-understood heat transfer characteristics on matierlas to be used. Herein, we investigate local and overall heat transfer characteristics of SLM processes and investigate the principal parameters related to the magnituge of a radiative heating power and its exposure time. We present how to exert their influence upon local melting and sequential solidification of copper powder bed. The local solid media reach a quasi-equilibrium state in even 1 ms with the incident powers of 50, 100, and 200 W. The anisotropic expansion of the molten pool is governed by a thermally-induced Marangoni flow. As the power is increased, the Marangoni factor increases linearly up to 853.7%. Consequential heat transfer characteristics tell us that unconditional input power should be avoided to prohibit the detrimental effect; the radiative heating power should be confined for thermalization of a target domain and for that preventing the evaporation of a material. These approaches from material science to heat transfer can be used to develop a platform for SLM processes guaranteeing its feasibility and applicability.

Use of an Abrasive Water Cavitating Jet and Peening Process to Improve the Fatigue Strength of Titanium Alloy 6Al-4V Manufactured by the Electron Beam Powder Bed Melting (EBPB) Additive Manufacturing Method

Metal components made by additive manufacturing have large inherent surface roughness, and, as such, their strength and fatigue life can be reduced significantly versus wrought products. In order to improve these properties, a novel mechanical surface treatment that introduces compressive residual stress while simultaneously reducing the surface roughness is proposed. The proposed treatment uses cavitation peening combined with an abrasive slurry. The impact of the kinetic energy-charged abrasive particles, induced by collapsing water cavitation vapor bubbles, produces compressive residual stress, while the abrasive reduces the surface roughness. Plane-bending fatigue tests were carried out to determine the effectiveness of this treatment on the fatigue life and strength of titanium alloy Ti6Al4V manufactured by electron beam melting. It was demonstrated that the fatigue strength of an as-built specimen was improved from 169 MPa to 280 MPa by the proposed treatment.

Thermal and Melting track Simulations of Laser Powder Bed Fusion (L-PBF)

Laser powder bed fusion (L-PBF) process involves with the construction of phase transformation, melting, and rapid solidification of weld metal powder bed which affects the properties and the microstructure of final parts, e.g. density, dimension, mechanical properties, void, porosity, and non-fully melted particle. The aims of this work were to study the effect of process parameters, e.g. laser power and scanning speed, on the temperature field and melt pool geometry and the characteristics of single melting track in the L-PBF process by using the commercial CFD software simulation Flow-3D (Flow-weld). The laser power, scanning speed, laser spot diameter, and layer thickness varied in this study were 120 W, 140 W, 0.4 m/s, 0.6 m/s, 0.8 m/s, 80 μm and 50 μm respectively. The results stated that at the lower scanning speed, the temperature field has a region of heat distribution larger than that of the higher one. The geometry of melt pools can be changed from ellipse shape to tear drop shape when the scanning speed is increased. The width and depth of laser melting track is increased when the higher laser power and lower scanning speed are applied. The void is found underneath the laser melting track when the scanning speed changes from 0.4 m/s to 0.6 m/s.

Experiments and simulations on solidification microstructure for Inconel 718 in powder bed fusion electron beam additive manufacturing

Previous research on the powder bed fusion electron beam additive manufacturing of Inconel 718 has established a definite correlation between the processing conditions and the solidification microstructure of components. However, the direct role of physical phenomena such as fluid flow and vaporization on determining the solidification morphology have not been investigated quantitatively. Here we investigate the transient and spatial evolution of the fusion zone geometry, temperature gradients, and solidification growth rates during pulsed electron beam melting of the powder bed with a focus on the role of key physical phenomena. The effect of spot density during pulsing, which relates to the amount of heating of the build area during processing, on the columnar-to-equiaxed transition of the solidification structure was studied both experimentally and theoretically. Predictions and the evaluation of the role of heat transfer and fluid flow were established using existing solidification theories combined with transient, three-dimensional numerical heat transfer and fluid flow modeling. Metallurgical characteristics of the alloy’s solidification are extracted from the transient temperature fields, and microstructure is predicted and validated using optical images and electron backscattered diffraction data from the experimental results. Simulations show that the pure liquid region solidified quickly, creating a large two-phase, mushy region that exists during the majority of solidification. While conductive heat transfer dominates in the mushy region, both the pool geometry and the solidification parameters are affected by convective heat transfer. Finally, increased spot density during processing is shown to increase the time of solidification, lowering temperature gradients and increasing the probability of equiaxed grain formation.

Three-dimensional finite element thermomechanical modeling of additive manufacturing by selective laser melting for ceramic materials

A model for additive manufacturing by selective laser melting of a powder bed with application to alumina ceramic is presented. Based on Beer–Lambert law, a volume heat source model taking into account the material absorption is derived. The level set method is used to track the shape of deposed bead. An energy solver is coupled with thermodynamic database to calculate the melting-solidification path. Shrinkage during consolidation from powder to liquid and compact medium is modeled by a compressible Newtonian constitutive law. A semi-implicit formulation of surface tension is used, which permits a stable resolution to capture the liquid/gas interface. The influence of different process parameters on temperature distribution, melt pool profiles and bead shapes is discussed. The effects of liquid viscosity and surface tension on melt pool dynamics are investigated. Three dimensional simulations of several passes are also presented to study the influence of the scanning strategy.

On the Microstructural Analysis of LFW Joints of Ti6Al4V Components Made via Electron Beam Melting

Additive Manufacturing (AM), applied to metal industry, is a family of processes that allow complex shape components to be realized from raw materials in the form of powders. The compaction of the powders is achieved by local melting of bed. Electron Beam Melting (EBM) is an additive manufacturing process in which a focalized electron beam is the heat source that allows the powders to be compacted. By EBM it is possible to realize complex shape components; this feature is of particular interest in titanium industry where numerous efforts are done to develop near net shape processes.One of the limits of EBM based AM process is the difficulty to realize large dimension parts. Due to this limit the study of joining processes of different parts is of great interest. In the present work the microstructure evolution of sheets of TI6Al4V made by EBM and joined by Linear Friction Welding (LFW) is analyzed in details.The bulk microstructure of the specimen is fine lamellar; lamellae are enclosed in alpha colonies. Different types of porosities are observed. In the joint a Thermo Mechanical Affected Zone (TMAZ) and Weld Bead (WB) are evident. In TMAZ a partial recrystallization occurs and the parent microstructure is deformed. Complete recrystallization occurs in WB whose structure is martensitic.

A Study on the Laser Spatter and the Oxidation Reactions During Selective Laser Melting of 316L Stainless Steel, Al-Si10-Mg, and Ti-6Al-4V

The creation of an object by selective laser melting (SLM) occurs by melting contiguous areas of a powder bed according to a corresponding digital model. It is therefore clear that the success of this metal Additive Manufacturing (AM) technology relies on the comprehension of the events that take place during the melting and solidification of the powder bed. This study was designed to understand the generation of the laser spatter that is commonly observed during SLM and the potential effects that the spatter has on the processing of 316L stainless steel, Al-Si10-Mg, and Ti-6Al-4V. With the exception of Ti-6Al-4V, the characterization of the laser spatter revealed the presence of surface oxides enriched in the most volatile alloying elements of the materials. The study will discuss the implication of this finding on the material quality of the built parts.